DO NOT MERGE: Only test up to 140 long file names.

test_mkdirs pretends to test creating many directories but it is in fact
testing for a very long file name.

Example directory name it was creating looks like:
/data/data/android.core.tests.runner/cache/harmony-test-FileTest/1/0...
(with lots of 0s).

TEST=cts-tradefed
  run cts -p android.core.tests.libcore.package.harmony_java_io
BUG=27370759

Change-Id: I7ee631de83425bbe91d28c7bd06d7f799ec5dc6d