Add clean-local target to clean pytest runner artifacts

The command finds all directories in bin/tests/system which contain an
underscore. Underscore indicates either a temporary directory (_tmp_), a
symlink to test artifacts (TESTNAME_MODULENAME), or a python-related
cache. Using underscore for a system test name is invalid and a hyphen
must be used instead.

bin/tests/system/Makefile.am

@@ -238,3 +238,6 @@ AM_LOG_FLAGS = -r
 $(TESTS): legacy.run.sh
 
 test-local: check
+
+clean-local::
+	-find $(builddir) -maxdepth 1 -type d -name "*_*" | xargs rm -rf