Skip signedness checks for type aliases of non-integer types

[johnschug]

Currently signedness checks are always generated for type aliases unless they are skipped manually via
TestGenerator::skip_signededness.
This pr skips generating signedness checks for type aliases unless the types they alias are known to be integer types.

[alexcrichton]

Thanks for this! It looks like AppVeyor CI may be failing? Also, mind running this against the libc crate to ensure the number of tests doesn't accidentally decrease?

[johnschug]

It looks like the AppVeyor failure was caused by #39 and is unrelated to the changes in this pr. I ran the libc tests locally with the updated ctest and had two failures (same as rust-lang/libc#1079 and rust-lang/libc#1080) which are unrelated to the changes here.

[alexcrichton]

Ok and to confirm, the number of tests didn't decrease? This seems like it could accidentally stop testing signedness which we're testing today

[johnschug]

Yes, it generates a total of 16205 tests both before and after the change.

[alexcrichton]

Ok great, thanks!