gh-39512: Add test for github format of doctest report
    
As in the title.

The feature was added in #36938 for
the purpose of showing GitHub annotations for test failures, but there
wasn't a doctest for it. This adds the test.

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview. (no change to documentation preview because private method.)

### ⌛ Dependencies

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URL: #39512
Reported by: user202729
Reviewer(s): Dima Pasechnik, user202729

Author: Release Manager

src/sage/doctest/forker.py

@@ -1247,6 +1247,17 @@ def _failure_header(self, test, example, message='Failed example:', extra=None):
             Hello there!
                 doctest_var = 42; doctest_var^2
             <BLANKLINE>
+
+        TESTS:
+
+        Test GitHub output format (used for GitHub Actions annotations)::
+
+            sage: DTR.options.format = 'github'
+            sage: print(DTR._failure_header(doctests[0], ex))
+            **********************************************************************
+            ::error title=Failed example:,file=.../sage/doctest/forker.py,line=12::Failed example:
+                doctest_var = 42; doctest_var^2
+            <BLANKLINE>
         """
         out = [self.DIVIDER]
         with OriginalSource(example):

src/sage/doctest/reporting.py

@@ -254,6 +254,14 @@ def _log_failure(self, source, fail_msg, event, output=None):
             Tests run before process (pid=1234) timed out:
             Output so far...
             **********************************************************************
+
+        TESTS:
+
+        Test GitHub output format (used for GitHub Actions annotations)::
+
+            sage: DTR.controller.options.format = 'github'
+            sage: DTR._log_failure(FDS, "Timed out", "process (pid=1234) timed out", "Output so far...")
+            ::error title=Timed out,file=.../sage/doctest/reporting.py::Output so far...
         """
         log = self.controller.log
         format = self.controller.options.format