no-std test harness

Unsorted TODOs

• Pre-test constructors and teardown may be relevant for embedded platforms that set up peripherals?
• Reorganize crate because the Test prefix is unnecessary for a lot of items only pulled in by the macro. Test description types could go in a submodule?
• Assertion macros that try/throw a Result instead of panic
  • Also unwrap/expect macros
• Support should_panic under the following conditions
  • It's the only test being run (or can be sorted so it runs last?)
  • The crate is allowed control over the panic handler (feature flag?)
• Clean up and pull the panic handler out of the test fn
  • ... and catch panics for all tests, not just should_panic ones
• Write meta tests that can assert failures are working
• Should run_tests just return normally instead of exiting?