test-guard.mk: Add file

Running "make clean" in some directories can break "make test" in
others. That notably happens by running clean somewhere in or below
$(TOPDIR)/src/python, which removes __init__.py files potentially needed
by unit and integration tests in $(TOPDIR)/test.

This commit makes the failure easily visible by introducing the notion
of "testabiltity" - if the tree is not testable, "make test" logs
exactly that in a clear error message and gives up.

Makefiles which need to check "testability" can use it as a
prerequisite. This commit does that with py-run.mk and jw-py-test.mk.

Testability is asserted by running "make", "make all" or "make test"
from $(TOPDIR). A successful toplevel build coincides with testability,
and leaves a $(TOPDIR)/dirs-all.done behind, which is why that file is
the perfect testability marker.

It is automatically created by a toplevel build and cleared by using the
"invalidate-testability" prerequisite. This commit makes target "clean"
depend on it in py-mod.mk. To be extended to other use cases / makefile
snippets as needed.

Signed-off-by: Jan Lindemann <jan@janware.com>

make/test-guard.mk

@@ -15,7 +15,7 @@ all:
 
 testability:
 	@test -f "$(TEST_GUARD)" || \
-		{ echo 'Error: Tree not testable -- run "make all" (or "make test") from the project\'s root to fix' >&2; exit 1; }
+		{ echo 'Error: Tree not testable -- run "make all" (or "make test") from project root to fix that!' >&2; exit 1; }
 
 invalidate-testability:
 	$(RM) -f "$(TEST_GUARD)"