glsl-mat-attribute: Use result of link status check.

Fixes unchecked return value defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
all.tests: split a plain test name into an array of arguments
fbo-drawbuffers: test masked clear
fbo-drawbuffers2-color: test masked clear