Treat bin/tests/system/_common as non-temp directory

The _common directory is a special case directory which contains shared
files for other system test directories. Make sure it's tracked in git
and not deleted during temporary directory cleanup.

bin/tests/system/Makefile.am

@@ -240,4 +240,4 @@ $(TESTS): legacy.run.sh
 test-local: check
 
 clean-local::
-	-find $(builddir) -maxdepth 1 -type d -name "*_*" | xargs rm -rf
+	-find -L . -mindepth 1 -maxdepth 1 -type d -name "*_*" -and -not -name "_common" -exec rm -rf {} \;