Improve namespace scoping between libcheckasm and the selftest

Currently most of the selftest functions also use the same checkasm name prefix as libcheckasm which everybody else should be calling. This makes it very hard to distinguish between which parts belong to the library itself and which parts belong to the selftest.

E.g. checkasm_check_generic(), checkasm_get_cpu_flags_x86(), etc - it would be clearer if we could easily see that checkasm_get_cpu_flags_x86() actually is the selftest's own function, which operates with its own definition of cpu flags, etc.

Not sure what would be a good namespace prefix to use instead; example would be good in one sense, but on the other hand, the checkasm selftest isn't so much about showing an example to others about how to write their tests (dav1d is a better example), but it's about exercising all the corner cases of checkasm itself. So perhaps selftest or something like that?