glcpp: Add tests for line continuation

glcpp: Add tests for line continuation

First we test that line continuations are honored within a comment, (as
recently changed in glcpp), then we test that line continuations can be
disabled via an option within the context. This is tested via the new support
for a test-specific command-line option passed to glcpp.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>