tests/graw: rename shaders from .sh to .txt

tests/graw: rename shaders from .sh to .txt

The vertex and fragment shaders are named foo.sh, while the geometry
shaders are named foo.txt. Since .sh can easily be confused with shell
scripts, let's rename the vertex and fragment shaders to .txt to match
the geometry shaders.

These tests aren't hooked up to run automatically, so there's no need to
update references to the file names.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16791>