[Piglit] Remove GLenum names from tests.

[Fabian Bieler]

This series replaces hard-coded GLenum names with calls to piglit_get_gl_enum_name.

Notes:

Some of the removed, hard-coded Glenum names had vendor, ARB, or EXT suffixes where piglit_get_gl_enum_name returns the suffix-less name.
This means that some subtests are renamed by this series.

In some cases string comparisons of command line arguments and GLenum names are replaced by integer comparison of piglit_get_gl_enum_from_name(cmd_line_arg) and GLenums.
This is to ensure that future updates to gl.xml don't necessitate updates to all.py.
However, these tests now abort on unknown command line arguments which were silently ignored, previously. I hope this is acceptable.

Fabian