piglit-build/build.py: Explicitly set build paramters

This explicitly turns on GL, GLES1, GLES2, GLES3, and GLX tests, and
turns of CL tests.

This is a change from the default which does not build GLES tests
builders.py: only try to clean build_dir if it exsits

This fixes a potential bug in the clean() methods.
build_support/__init__.py: Force build, clean, and test order

The order must be clean first, then build, finally test, although steps
may be skipped, they must be run in order. This patch changes a
dictionary to a list of tuples to ensure that an ordered data type is
used.