[Piglit] Cleanups for gen_const_builtine_equal_tests.py

[Dylan Baker]

This series does some small cleanups and optimizations to this test
generator.

Patch 1: converts to using mako. This is cleaner and easier to read than
         the previous implementation
Patch 2: Use a main() function
Patch 3: simplify and combine loops with enumerate
Patch 4: small cleanups for style