CommonPool should be configurable to have a minimum number of threads

[svd27]

Currently CommonPool sets the number of threads in its pool to
max(NumberofCpus - 1,1)
this leads that on travis a lot of tests are failing for me. it would be nice to have something like a system property:
kotlinx.experimental.coroutines.commonpool.minThreads
which would allow to override that for unit tests

[fvasco]

See #261

[qwwdfsad]

For tests, it makes sense to provide separate pool and use it, you can see such pattern in our stress tests. CommonPool is a singleton, so changing its size via global system property may be error prone (especially for tests), so I would not recommend that.

Nevertheless, this is a valid issue (e.g. see #321), so we definitely will address it