shader_runner: Add command line option to ignore uniform if it isn't found in shader

Piglit makes the choice of failing quickly in the event that a test case is potentially malformed. When performing something like fuzz testing for compilers, unused uniforms can be declared. When the shader is compiled, most compilers will attempt to optimize away the unused uniforms, causing shader_runner to fail immediately if attempting to load a value into them.

This PR adds a command line argument to shader_runner, -ignore-missing-uniforms, that will simply ignore [test] directives to load a uniform or attribute if said uniform/attrib cannot be found in the shader instead of immediately failing.