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>
diff --git a/tests/Makefile.inc b/tests/Makefile.inc
index ca974a9..027091c 100644
--- a/tests/Makefile.inc
+++ b/tests/Makefile.inc
@@ -39,6 +39,8 @@
 
 TEST_CFLAGS += -std=gnu11 -Os -ffunction-sections -fdata-sections -fno-builtin
 
+TEST_CFLAGS += -D__TEST__
+
 # Checkout Cmocka repository
 forgetthis:=$(shell git submodule update --init --checkout 3rdparty/cmocka)