New `--start-at` flag to test runner to complement the existing `--failfast`

[Sam Clegg]

If you work on emscripten and you are trying to find and fix all failing tests related to your current branch, these two flags can be useful together:

1. Run the test suite with `--failfast -j1`.  This will run the whole suite one test at a time.
2. When the first failure occurs you can stop and fix it.
3. Re-run with `--start-at <last-failing-test> --failfast -j1` to continue where you left off.

cheers,

sam