test: add test to track additional bugs

Several related bugs that stem from a diff that
contains both deletions and additions. Specifically,
the line numbers aren't counted correctly, leading
to
- issue URL can't be inserted because it can't find
  the right line in the latest file
- generated issue references the wrong line number
- closed issue references the wrong line number

See GitHub issue #236

The last item might not have any actual impact as
(I think) it's just informational. But it'd still
be better if it reported the correct line number
of the deletion, which necessarily has to be
relative to the _old_ file's line number, not the
updated file's.

As there is no solution in place yet for these
bugs, the unittest is marked as an expected failure

tests/test_edit_py.diff

@@ -0,0 +1,15 @@
+diff --git a/example_file.py b/example_file.py
+index 6b0c6cf..b37e70a 100644
+@@ -1,3 +1,9 @@
++def imaginative_string():
++    return 'figment of my imagination'
++
+ def hello_world():
+-    # TODO: Come up with a more imaginative greeting
+-    print('Hello world')
+\ No newline at end of file
++    print(f'Hello {imaginative_string()}')
++
++    # TODO: Do more stuff
++    #  This function should probably do something more interesting
++    #  labels: help wanted

tests/test_new_py.diff

@@ -0,0 +1,10 @@
+diff --git a/example_file.py b/example_file.py
+new file mode 100644
+index 0000000..525e25d
+--- /dev/null
++++ b/example_file.py
+@@ -0,0 +1,3 @@
++def hello_world():
++    # TODO: Come up with a more imaginative greeting
++    print('Hello world')
+\ No newline at end of file

tests/test_process_diff.py

@@ -11,12 +11,17 @@ from main import process_diff
 
 
 class IssueUrlInsertionTest(unittest.TestCase):
+    _original_addSubTest = None
+    num_subtest_failures = 0
     orig_cwd = None
     tempdir = None
     diff_file = None
     parser = None
 
     def _setUp(self, diff_files):
+        # reset counter
+        self.num_subtest_failures = 0
+
         # get current working directory
         self.orig_cwd = os.getcwd()
 
@@ -44,6 +49,8 @@ class IssueUrlInsertionTest(unittest.TestCase):
         output = io.StringIO()
         # process the diffs
         self.raw_issues = process_diff(diff=self.diff_file, insert_issue_urls=True, parser=self.parser, output=output)
+        # store the log for later processing
+        self.output_log = output.getvalue()
         # make sure the number of issue URL comments inserted is as expected
         self.assertEqual(output.getvalue().count('Issue URL successfully inserted'),
                          expected_count,
@@ -51,6 +58,19 @@ class IssueUrlInsertionTest(unittest.TestCase):
                              '\nProcessing log\n--------------\n'+output.getvalue()
                                 if output_log_on_failure else None))
 
+    def _addSubTest(self, test, subtest, outcome):
+        if outcome:
+            self.num_subtest_failures+=1
+        if self._original_addSubTest:
+            self._original_addSubTest(test, subtest, outcome)
+
+    def run(self, result=None):
+        if result and getattr(result, "addSubTest", None):
+            self._original_addSubTest = result.addSubTest
+            result.addSubTest = self._addSubTest
+
+        super().run(result)
+
     # this test can take a while and, as far as TodoParser is concerned,
     # redundant with the tests of test_todo_parser, so enable the means
     # to skip it if desired
@@ -60,6 +80,34 @@ class IssueUrlInsertionTest(unittest.TestCase):
         self._setUp(['test_new.diff'])
         self._standardTest(80)
 
+    # See GitHub issue #236
+    @unittest.expectedFailure
+    def test_line_numbering_with_deletions(self):
+        self._setUp(['test_new_py.diff', 'test_edit_py.diff'])
+        with self.subTest("Issue URL insertion"):
+            # was issue URL successfully inserted?
+            self._standardTest(1, False)
+        with self.subTest("Issue insertion line numbering"):
+            # make sure the log reports having inserted the issue based on the
+            # correct line numbering of the updated file
+            self.assertIn("Processing issue 1 of 2: 'Do more stuff' @ example_file.py:7",
+                          self.output_log)
+        with self.subTest("Issue deletion line numbering"):
+            # make sure the log reports having closed the issue based on the
+            # correct line numbering of the old (not the updated!) file
+            self.assertIn("Processing issue 2 of 2: 'Come up with a more imaginative greeting' @ example_file.py:2",
+                          self.output_log)
+
+        if self.num_subtest_failures > 0:
+            self.fail(
+                '\n'.join([
+                '',
+                'One or more subtests have failed',
+                'Processing log',
+                '--------------',
+                ''])+
+                self.output_log)
+
     # There is a known bug related to this issue, so until it's resolved
     # this is an expected failure.
     # See #225 and #224