googletest: enable failure on uninstantiated tests

Similar to the change in
https://aomedia-review.googlesource.com/c/aom/+/115162.
This currently is a warning, but the tree should be clean now in the
default x86-64 configuration so we can use it to prevent regressions and
find any remaining issues in other configurations.

BUG=b/159031844

Change-Id: I097537ff018668492d37164fdba5edd241dc5dbe
2 files changed