Split String.split test into multiple; shard slow "fast path" test

Originally, String.split test was a single funstion testing everything.
This CL splits this function into a series of smaller test functions,
each testing specific case.

Also, shared slow "fast path" test (was running ~50s) before into
five shards, so at maximum slowest test shard now is around 14s.

Bug: 207784876
Test: atest CtsLibcoreOjTestCases:test.java.lang.String.Split
Change-Id: I7df984e02543de2d2c86307ee075f0073a2986a2
1 file changed