Move doctests checking to own target in preorder traversal

As spelled before this patch, the doctests were running after the more
time-consuming `case_utils` descent.  This lets the tests be called on
their own, and also lets a failing doctest fail the CI job quicker.

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

tests/Makefile

@@ -23,6 +23,7 @@ all: \
 .PHONY: \
   all-case_utils \
   check-case_utils \
+  check-doctest \
   check-isomorphic_diff \
   check-mypy \
   download
@@ -57,13 +58,9 @@ all-case_utils: \
 # These check calls are provided in preferred run-order.
 check: \
   check-mypy \
+  check-doctest \
   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 \
@@ -76,6 +73,14 @@ check-case_utils: \
 	  --directory case_utils \
 	  check
 
+check-doctest: \
+  .venv.done.log
+	source venv/bin/activate \
+	  && pytest \
+	    --doctest-modules \
+	    --log-level=DEBUG \
+	    $(top_srcdir)/case_utils
+
 check-isomorphic_diff: \
   .venv.done.log
 	$(MAKE) \