Move tests to a (non-published) sub-crate

[fitzgen]

And move the build.rs to that sub-crate, since it is only used for generating tests.

This should help cargo better-parallelize builds of crate graphs that include cpp_demangle.

This requires somethign like:

• Create a new cpp_demangle_tests crate located at crates/tests
• Move all the tests into there
• Move build.rs to that crate
• Update CONTRIBUTING.md to mention cargo test --all for testing