Verify that all backends are loaded before running MessageComposer crypto tests

The crypto test is crashing on CI due to smime() returning a null pointer.
Looking into Kleo the only reasons seems to be that the support is missing in
Kleo or GpgMe, but we don't know exactly what's wrong.

This adds a check to setupEnv() to verify all backends and protocols are
initialized correctly or print the reasons why not.