util/lint: Add lint and checkpatch coverage for tests/ dir

Signed-off-by: Jan Dabros <jsd@semihalf.com>
Change-Id: I8018b75844e630c9ed46c8bc48f2aa1634bf3369
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43511
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>

util/lint/lint-stable-009-old-licenses

@@ -23,7 +23,7 @@ _shipped$|\
 "
 
 if [ -z "$HEADER_DIRS" ]; then
-	HEADER_DIRS="src util"
+	HEADER_DIRS="src util tests"
 fi
 
 LC_ALL=C export LC_ALL