[Piglit] Make options a global variable

[baker.dylan.c at gmail.com]

One of the awkward things in piglit is passing the options object around
everyplace we need it, which is a lot of places. This creates a lot of
code, and a lot of passing, and a lot of places for something to fail.
This small series turns options into a global variable, which can be
mutated (including reset to stock). This helps to simplify options
handling a bit, and will make using options in new places simpler.