framework/exectest.py: Mark several Test methods as not API

Python doesn't have private or protected methods or members, instead it
subscribes to the "we're all consenting adults" theory of programming.
However, it does have conventions for marking methods as "don't use
these, they are subject to change". Any method marked with a leading
underscore is marked as "This is not a safe API, it's not guaranteed to
not change". These methods act as a sort of pseudo-private method.

This patch changes Test.interpret_result to Test._interpret_result,
Test.check_for_skip_scenario() to Test._check_for_skip_scenario(), and
Test.get_command_result to Test._run_command(). These methods shouldn't
be called externally. It also changes the PIGLIT_PLATFORM constant to
_PIGLIT_PLATFORM, again marking that it is meant for use in the module
exclusively.
framework/exectest.py: update docstrings

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>