Shard slow TestNg tests for art buildbot

This CL splits tests from the list below into smaller test targets to
make art buildbot run tests faster:

 * test.java.math.BigInteger
 * test.java.security.KeyAgreement
 * test.java.lang.StrictMath

Bug: 229830804
Test: art/tools/run-libcore-tests.sh --mode=host
Test: art/tools/run-libcore-tests.sh --mode=device
Change-Id: I87ac94583676a3b540a25710c5f22b58230e2e5c
1 file changed