cylc test-battery: use global process pool size

Use `process pool size` in site/user global configuration as the
argument for the `-j` option for `prove`.
Use a symbolic link to link parsec tests into the `tests/` sub-directory
so all test files can be found under the same roof.
cylc test-battery: update usage

Update default test location.
Update concurrency usage.
Merge pull request #1237 from matthewrmshin/test-battery-nproc

cylc test-battery: use global process pool size