test: one more fail-under test

I saw this bug against pytest-cov, and thought it was my fault:
pytest-dev/pytest-cov#601, but pytest-cov has
their own comparison and reporting code for fail-under.  Without the
plugin, you won't get a message like "total of X is less than X."  This
test helped confirm that, so we'll keep it.

tests/test_cmdline.py

@@ -1214,6 +1214,7 @@ def test_fail_under(
             "Coverage failure: total of 20.5 is less than fail-under=20.6\n"),
         (20.12345, "report --fail-under=20.1235 --precision=5", 2,
             "Coverage failure: total of 20.12345 is less than fail-under=20.12350\n"),
+        (20.12339, "report --fail-under=20.1234 --precision=4", 0, ""),
     ])
     def test_fail_under_with_precision(self, result: float, cmd: str, ret: int, msg: str) -> None:
         cov = CoverageReportingFake(report_result=result)