[tests] Re-added debug mode in tests

The EXTRA_FLAGS needs some clean before running tests.
This commit allows to compile twice for this situation, but it is not perfect
because EXTRA_CFLAGS can still cause some problems. The tests Makefile should
be able to detect this and to group builds and tests with each own EXTRA flags.

tests/Makefile

@@ -11,7 +11,9 @@ debugtest:
 	$(MAKE) -C debug-mode -j; \
 	debug-mode/debug-tests.out
 
-runtests: clean tests
+runtests: debugtest
+	$(MAKE) clean
+	$(MAKE) tests
 	set -e; \
 	for i in *-tests/Makefile; do \
 		echo '==== Running:' $$(dirname $$i); \