Add tests for internal/mkcgo

[qmuntal]

This PR adds unit tests for the internal/mkcgo package. The coverage is 100% (if that means somethinf 😄):

go test -cover ./internal/mkcgo 
ok      github.com/golang-fips/openssl/v2/internal/mkcgo        7.642s  coverage: 99.6% of statements

I've refactored some APIs to make to make them easier to test, added some additional validations, and fixed some bugs (which shims.h was not exercising) along the way.

Test for cmd/mkcgo will come in a follow-up PR.