tests: fix checkstyle-python

Add --check to black to pass back error value and fail checkstyle if
a reformat would occur.

Type: fix
Fixes: d9b0c6fbf7aa5bd9af84264105b39c82028a4a29
Signed-off-by: Klement Sekera <klement.sekera@gmail.com>
Change-Id: I9a1fbe224929fc461ff833a589f73ca06e7cc9d6
diff --git a/test/Makefile b/test/Makefile
index b416bc3..08c6eab 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -351,7 +351,7 @@
 .PHONY: checkstyle-python-all
 checkstyle-python-all: $(PIP_INSTALL_DONE)
 	@bash -c "source $(VENV_PATH)/bin/activate &&\
-		black -t py39 --diff $(WS_ROOT) ||\
+		black -t py39 --check --diff $(WS_ROOT) ||\
 		(echo \"*************************************************************************\" &&\
 		echo \"* Test framework PEP8 compliance check FAILED (maybe: make fixstyle-python)\" &&\
 		echo \"*************************************************************************\" &&\