Enable doctests on modules

This patch enables review of Python code inlined in the `case-utils`
modules' docstrings.

At the moment, no doctests are written, so this patch has no effect on
the test suite.  It is provided before tests are written to demonstrate
a non-impact on CI.

A different patch series will be exercising doctests.

References:
* https://docs.pytest.org/en/7.1.x/how-to/doctest.html

Signed-off-by: Alex Nelson <[email protected]>

Author: ajnelson-nist

tests/Makefile

@@ -59,6 +59,11 @@ check: \
   check-mypy \
   check-isomorphic_diff \
   check-case_utils
+	source venv/bin/activate \
+	  && pytest \
+	    --doctest-modules \
+	    --log-level=DEBUG \
+	    $(top_srcdir)/case_utils
 	source venv/bin/activate \
 	  && pytest \
 	    --ignore case_utils \