include/rules.h: Add ENV_TEST definition

Some functions/macros like assert() require redefinition for testing
purposes. ENV_TEST is introduced to make it possible without using
bypass hacks.
This patch also adds a global __TEST__ define to TEST_CFLAGS for
all test targets in order to enable ENV_TEST.

Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Change-Id: Ib8f2932902a73a7dbe181adc82cc18437abb48e8
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51803
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/include/rules.h

@@ -3,6 +3,12 @@
 #ifndef _RULES_H
 #define _RULES_H
 
+#if defined(__TEST__)
+#define ENV_TEST 1
+#else
+#define ENV_TEST 0
+#endif
+
 #if defined(__TIMELESS__)
 #define ENV_TIMELESS 1
 #else