Allows test/suite/net.js test to be rerun in succession.
Simplifies "tap" script.
Converts many tests to use the tap tester.
Fixes test 419 to use tap.count.
Merge #417 r=@tcr: Normalizes all tests to use the tap module.

Most tests should now use the "tap" module located at `src/tap.js`. This is a bare-bones tester that encapsulates testing logic without introducing too many failure points for tests. (That is, if you can build a JS runtime that evaluates the `tap.js` script, then you should be able to make use of the tests.)