Convert some tests to use PIGLIT_GL_TEST_CONFIG_* macros.

These tests aren't doing anything fancy in main(), so they can use
PIGLIT_GL_TEST_CONFIG_BEGIN and PIGLIT_GL_TEST_CONFIG_END.
framework: allow locals to be declared in piglit config block.
framework: parse piglit core args during piglit_gl_test_config_init()

This makes it safe to examine argc and argv between
PIGLIT_GL_TEST_CONFIG_BEGIN and PIGLIT_GL_TEST_CONFIG_END, since
piglit fragmework arguments have already been stripped away.

That in turn means that the following tests can use
PIGLIT_GL_TEST_CONFIG_{BEGIN,END} instead of having to declare their
own main() functions:

- spec/arb_texture_buffer_object/formats.c
- spec/gl-3.0/required-renderbuffer-attachment-formats.c
- spec/gl-3.0/required-sized-texture-formats.c
- spec/gl-3.0/required-texture-attachment-formats.c
- spec/gles-3.0/oes_compressed_etc2_texture-miptree.c