Add tests for questionable API use

[apasel422]

The following can and should panic:

• Calling Gc::deref within force_collect
• Calling Gc::clone within force_collect
• Calling Gc::new within GcState::drop

The following would ideally panic, but might have to abort due to double panic, unspecified TLS drop order, or the inability to access TLS values during or after their destruction:

• Calling Gc::deref within GcState::drop
• Calling Gc::clone within GcState::drop
• Calling Gc::new within GcState::drop

The following currently panic, but could also be made to work:

• Calling force_collect within force_collect (would do nothing if nightly's RefCell::try_borrow_mut method indicates that a borrow is already outstanding)
• Calling Gc::new within force_collect