compiletest: Don't limit all suites on Android

On Android we only have one test thread for supposed problems with concurrency
and the remote debugger. Not all of our suites require one concurrency, however,
and suites like compile-fail or pretty can be much faster if they're
parallelized on Travis.

This commit only sets the test threads to one on Android for suites which
actually run code, and other suites aren't tampered with.

Rollup of 25 pull requests

- Successful merges: #37149, #38491, #38517, #38559, #38587, #38609, #38611, #38622, #38628, #38630, #38631, #38632, #38635, #38647, #38649, #38655, #38659, #38660, #38662, #38665, #38671, #38674, #38676, #38693, #38695
- Failed merges: #38657, #38680

Rollup of 25 pull requests

- Successful merges: #37149, #38491, #38517, #38559, #38587, #38609, #38611, #38622, #38628, #38630, #38631, #38632, #38635, #38647, #38649, #38655, #38659, #38660, #38662, #38665, #38671, #38674, #38676, #38693, #38695
- Failed merges: #38657, #38680

Rollup of 25 pull requests

- Successful merges: #37149, #38491, #38517, #38559, #38587, #38609, #38611, #38622, #38628, #38630, #38631, #38632, #38635, #38647, #38649, #38655, #38659, #38660, #38662, #38665, #38671, #38674, #38676, #38693, #38695
- Failed merges: #38657, #38680

Rollup of 25 pull requests

- Successful merges: #37149, #38491, #38517, #38559, #38587, #38609, #38611, #38622, #38628, #38630, #38631, #38632, #38635, #38647, #38649, #38655, #38659, #38660, #38662, #38665, #38671, #38674, #38676, #38693, #38695
- Failed merges: #38657, #38680