parse test macros from `gcc -E -dM` output instead of the raw header

parse test macros from `gcc -E -dM` output instead of the raw header

previously we had no way intelligently parsing out conditional
defines such as:

```c
```

we would parse out TB_XYZ twice and overwrite the first value. this
was breaking tests as we now, for example, have TB_BOLD defined
conditionally.

we are now parsing the output of `gcc -E -dM` instead which give us a
simple list of macro key-vals after the preprocessor has executed.