Ensure each test block starts in the original cwd.

Forking normally takes care of this.  This ensures it works with
parallelism off.
Each test block has a pristine %ENV.

Again, forking normally takes care of this.  Now it'll work the same way
with or without parallel.
Git ignore MYMETA.json as well.
Merge pull request #102 from schwern/feature/isolate_tests

Isolate tests changes to %ENV or cwd when parallel is off