Speed up integration tests

[elliotchance]

It's nice to have one C file per function, but where there is little change in syntax between the files (like testing all the math.h functions) this just means the tests take a long time.

We should consider reducing these into a single test file, like tests/math.c which will make testing much faster.

[zoeyfyi]

The integration tests could also probably be made to run in parallel.

[elliotchance]

Parallel would be better, but I still think that running through the entire build chain just to test if a single function outputs the correct value is proving to be overkill.

After all, these are integrations tests so they are made to test a bunch of stuff in a single round.