tests: Add some basic warnings and fix resulting issues

The current test framework builds the test code without any warnings at
all, which isn't great -- we have already slipped in some cases of
non-void functions not returning a defined value, for example. It would
likely be overkill to try to use all the same warnings we use for normal
coreboot code (e.g. some stuff like -Wmissing-prototypes makes cmocka's
__wrap_xxx() mock functions unnecessarily cumbersome to work with, and
other things like -Wvla may be appropriate for firmware but is probably
too aggressive for some simple test code). Therefore, let's just add
some of the stuff that points out the most obvious errors.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I4d9801f52a8551f55f419f4141dc21ccb835d676
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42259
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jan Dabros <jsd@semihalf.com>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>

tests/Makefile.inc

@@ -25,7 +25,14 @@ TEST_CFLAGS = -include $(src)/include/kconfig.h \
 TEST_CFLAGS += -I$(testsrc)/include/mocks -I$(testsrc)/include
 
 TEST_CFLAGS += -I$(src)/include -I$(src)/commonlib/include \
-	-I$(src)/commonlib/bsd/include -I$(src)/arch/x86/include \
+	-I$(src)/commonlib/bsd/include -I$(src)/arch/x86/include
+
+# Note: This is intentionally just a subset of the warnings in the toplevel
+# Makefile.inc. We don't need to be as strict with test code, and things like
+# -Wmissing-prototypes just make working with the test framework cumbersome.
+# Only put conservative warnings here that really detect code that's obviously
+# unintentional.
+TEST_CFLAGS += -Wall -Werror -Wundef -Wstrict-prototypes
 
 # Path for Kconfig autoheader
 TEST_CFLAGS += -I$(dir $(TEST_KCONFIG_AUTOHEADER))