The CI now runs C backend tests in addition to compiling them. It uses
-std=c99 -pedantic -Werror in order to catch non-conformant C code.

This necessitated disabling a test case that caused a C compile error,
in addition to disabling a handful of warnings that are already being
triggered by Zig's C backend output for the behavior tests.

The upshot is that I was able to, very cleanly, integrate the C backend
tests into the build system, so that it communicates via the test runner
protocol along with all the other behavior tests.