r300g: Use PATH_MAX instead of limiting ourselves to 100 chars.

r300g: Use PATH_MAX instead of limiting ourselves to 100 chars.

When built with Gentoo's package manager, the Mesa source directory
exists seven directories deep. The path to the .test file is too long
and is silently truncated, leading to a crash. Just use PATH_MAX.

Cc: 10.4, 10.5 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.gentoo.org/show_bug.cgi?id=540970
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>