Factor out most C code from mkmutest to mutest.c

Factor out most C code from mkmutest to mutest.c

This makes easier to improve the tester code, without having to code it as
a shell script embedded string. Now the only generated code is the one
that runs the test suites. You can also implement your own test suites
execution function; just write it in a separated C module and don't use
the mkmutest generator.