piglit: no-isolation shader_runner support

We could greatly reduce the piglit run overhead if we did proper no-isolation support for shader_runner. My thought is that we need to:

• Parse the extension requirements for shader_runner tests from the XML.
• Make test groups of multiple shader_runner tests that have identical requirements.
• Ask shader_runner to print subtest info.
• Parse subtest output and map that to test statuses.
• Ignore top-level piglit result.