test_util: Add assertion inside weakly-linked run_test()
test_util: Add assertion inside weakly-linked run_test()

The weakly-linked definition of run_test() should never be executed.
When it does, it's usually a symptom of an issue with the build of the
real test.

Add an assertion to make the error condition easier to detect and
resolve.

BUG=b:243700149
TEST=Run host test with bad definition of run_test()
     =>Error message is printed on console.
BRANCH=none
LOW_COVERAGE_REASON=New function is only called from invalid builds.

Signed-off-by: Andrea Grandi <f017805e76e0e7a74072a3bac1b6e29e65eedeab@google.com>
Change-Id: Iaf6c8377618944efc5ad4d4557ed4c231c09be09
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3887968
Code-Coverage: Zoss <345cf107e82c70959d46f59d98bddc98ce458aff@prod.google.com>
Reviewed-by: Bobby Casey <a3c107f0b4efaee07148ab0813ba992bba562a69@google.com>
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec