tests for GCC internal functions

[Martin Sebor]

Is there a setup for writing and running as part of the test
suite unit tests that exercise internal GCC functions like
error and warning?  I ask because of a couple of bugs that
were recently reported for the %s directive in GCC's
diagnostics (81859 and 81586) that were only caught by running
valgrind on GCC, but that could have been caught by GCC's own
tests for these APIs if they existed.

I can't find any except some in the gcc.dg/plugin directory
(though they weren't written to exercise the functions but
rather the plugin interface).  Would adding a plugin for
this sort of thing be the right way to do it?

Thanks
Martin