Fix regex for visibility attribute tests

Fix regex for visibility attribute tests

The previous regex of `not supported` was very specific to a particular
compiler (Clang 3.4+). As Clang isn't the only compiler that throws a
warning (but otherwise succeeds) when a visibility isn't supported, make
the regex more generic to hit all such cases.

Testing on Compiler Explorer shows that looking for the string
`visibility` has a better hit rate. `attribute` is perhaps more
dangerously generic, and `hidden`/`internal` doesn't always show up in
warning messages when the visibility attribute itself isn't available.