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/commonlib/region-test.c

@@ -69,6 +69,8 @@ static ssize_t mock_readat(const struct region_device *rdev, void *buffer,
 	ssize_t ret = mock();
 	if (!ret)
 		return size;
+	else
+		return ret;
 }
 
 static ssize_t mock_writeat(const struct region_device *rdev, const void *buffer,
@@ -82,6 +84,8 @@ static ssize_t mock_writeat(const struct region_device *rdev, const void *buffer
 	ssize_t ret = mock();
 	if (!ret)
 		return size;
+	else
+		return ret;
 }
 
 static ssize_t mock_eraseat(const struct region_device *rdev, size_t offset, size_t size)
@@ -93,6 +97,8 @@ static ssize_t mock_eraseat(const struct region_device *rdev, size_t offset, siz
 	ssize_t ret = mock();
 	if (!ret)
 		return size;
+	else
+		return ret;
 }
 
 struct region_device_ops mock_rdev_ops = {
@@ -155,8 +161,6 @@ static void rdev_mock_defaults(void)
 
 static void test_rdev_success(void **state)
 {
-	struct region_device child;
-
 	expect_value(mock_mmap, size, region_device_sz(&mock_rdev));
 
 	rdev_mock_defaults();