Allow MockVM to run GC

[qinsoon]

This PR made some changes to MockVM to allow us to run GC in the tests.

• Add features mock_test_side_metadata and mock_test_header_metadata. We only test with mock_test_side_metadata (as some plans don't run with header metadata).
• No longer use lock for the MockVM instance (otherwise it will cause deadlock when we reentry MockVM)
• Statically initialize a MMTK instance for mock tests.
• Mock tests now use mock_api to create a mutator. This registers the mutator to mock VM, and can be blocked for GC.
• Mock tests now properly initialize GC threads, and handle panics in GC threads.
• Minor changes to print more information for mmap failures.

[wks]

std::sync::ReentrantLock is an unstable feature. But a third-party crate parking_lot has an implementation of ReentrantMutex. If that is helpful, we can add the parking_lot as a dependency when the mock_test feature is enabled.