tests/Makefile.inc: Enable support for multiple test groups

Until now output of all test groups run in single unit test were
saved in the same file which caused Jenkins to fail because
of existence of multiple root XML elements.
Now each test group is saved to its own file containing its name
at the end of the filename.

Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Change-Id: I21ba512073bc8d8693daad8a9b86d5b076bea03f
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51281
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
diff --git a/tests/Makefile.inc b/tests/Makefile.inc
index 1e260fa..ca974a9 100644
--- a/tests/Makefile.inc
+++ b/tests/Makefile.inc
@@ -148,7 +148,7 @@
 
 ifeq ($(JUNIT_OUTPUT),y)
 $(alltests): export CMOCKA_MESSAGE_OUTPUT=xml
-$(alltests): export CMOCKA_XML_FILE=$(testobj)/junit-$(subst /,_,$^).xml
+$(alltests): export CMOCKA_XML_FILE=$(testobj)/junit-$(subst /,_,$^)-%g.xml
 endif
 
 $(alltests): $$($$(@)-bin)