__LINE__ can be inconsistent when using different compilers. This patch
changes the test runner to do a simple string find/replace of the test
source file instead of looking for the line where the reference string
starts.
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33980>