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/py-run.mk

@@ -1,5 +1,6 @@
 include $(JWBDIR)/make/defs.mk
 include $(JWBDIR)/make/py-defs.mk
+include $(JWBDIR)/make/test-guard.mk
 
 EXE                         ?= $(firstword $(wildcard main.py runme.py test.py *.py))
 EXE_ARGS                    ?=
@@ -9,7 +10,7 @@ all:
 install:
 clean:
 distclean:
-test: all
+test: all testability
 
 run:
 	$(PYTHON_RUNNER) $(EXE) $(EXE_ARGS)