Add tests to ensure no dylibs exist in release builds

Reference: https://github.com/mono/mono/issues/13107

The App Store doesn't accept apps with dylibs in them, which means that no release builds should ever contain dylibs.

We build plenty of release builds, both in the mtouch tests and when running device tests. We should add asserts to some (all?) of these that verify that there are no dylibs in the .app.