Allow use of interactive debugger when running tests in watch mode

[mattbrictson]

Before, mt --watch would take full control of stdin to listen for keyboard commands. This prevented the use of an interactive debugger when a test is being executed.

This PR changes the event loop implementation so that control of stdin is relinquished when running tests. This allows an interactive debugger to be used while tests are running.

Specifically:

• Move stdin reader (i.e. $stdin.getc) out of a background thread and into the main event loop.
• $stdin.getc blocks until a character is typed.
• When the file system listener background thread detects a change, raise an exception in the main event loop to break out of $stdin.getc and proceed to run the tests.