Break Travis tests into smaller chunks

[aronisstav]

These days Travis tests take ~21 minutes to complete.

https://travis-ci.org/parapluu/Concuerror/builds/192348544

Maybe we could break down the tests target into two or three groups (e.g. per suite) to reduce this time.

Probably not worth it if it doesn't halve the total execution time.

[aronisstav]

It seems that travis runs 5 tests concurrently (tweet is old but should be accurate):

https://twitter.com/travisci/status/651856122559774722

The total execution time of the same batch of tests was 1 hr 18 min.

If it was perfectly parallelized it would need 78 min/5 = 15 min, so not half the time.

Experiment still needed, though.

[aronisstav]

From: https://travis-ci.org/parapluu/Concuerror/builds/284682153

Ran for 17 min 49 sec
Total time 1 hr 44 min 21 sec

Good enough.