Re-implement StopwatchTests using alternative approaches for the timed wait
Replicate DelayedConstraintTests using different methods for time delay
Eliminate tests for max delay
AutoResetEvent seems to provide most consistent delay, so remove other delay alternatives
Change approach to testing early return
Increase tolerance for stopwatch comparison
Treat Invalid  tests as Failed rather than skipped
Fix error in engine tests
Merge pull request #303 from nunit/issue-229

Issue 229 - Improvements to the timing tests
Merge pull request #302 from nunit/issue-271

Treat Invalid  tests as Failed rather than skipped
Merge branch 'master' into issue-288