Keep an unrecognised status inside its code span

`status_phrase` renders a status it does not know verbatim, deliberately:
`argparse`'s `choices=` would exit 2 on an unexpected value and the log --
the entire reason this script exists -- would never be posted.

But the verbatim value lands in a code span inside a **bold** header, so a
backtick in it closes the span early and the rest renders as markdown:

    --status 'x` **loud** `y'
    -> **`tests` finished with status `x` **loud** `y`**

Nothing hostile is expected: the value comes from `${{ job.status }}` or a
hand-written flag, both written by whoever wrote the workflow. It is worth
closing anyway because this repo is public and four others consume the
script as a composite action, so a branch name or a matrix value could
reach this argument later without anyone revisiting this function.

Backticks are removed rather than escaped -- there is no escape for a
backtick inside a code span, only a wider fence, and the status is a short
word rather than something whose exact bytes matter.

Found in the cold re-read of #10, not by the suite, so the test that
covers it was proved to fail without the fix.

Co-authored-by: bit <bit@das-labor.org>

test_report_job_log.py

@@ -313,6 +313,15 @@ class TestStatusPhrase(unittest.TestCase):
         self.assertEqual(report_job_log.status_phrase("weird"),
                          "finished with status `weird`")
 
+    def test_a_backtick_in_an_unknown_status_cannot_escape_the_code_span(self):
+        """The verbatim value sits in a code span inside a **bold** header, so
+        a backtick in it would close the span and let the rest render as
+        markdown. `${{ job.status }}` is workflow-author-controlled rather than
+        hostile, but this script is public and shared by four repos."""
+        phrase = report_job_log.status_phrase("x` **loud** `y")
+        self.assertEqual(phrase, "finished with status `x **loud** y`")
+        self.assertEqual(phrase.count("`"), 2)
+
     def test_default_constant_is_failed(self):
         """Named so that changing it is a deliberate act, not a typo."""
         self.assertEqual(report_job_log.DEFAULT_STATUS, "failed")