The test currently runs a busy loop that does string operations 1
billion times, when really it just needs something that takes more than
2 seconds of CPU time. On the other hand, there's another test that uses
the same method to run only 100 string operations for a different test.
Add a second parameter to have it exit early if enough CPU time has been
used, and use it in the first test.
Bug: T220685
Change-Id: I32309f049bcfb2e1310651818ff8674f0a311f40
