health@2.0 VTS test: fix flaky test

An implementation may or may not invoke the callback
immediately when registerCallback is called, causing
a race in the test. Fix the test by waiting for the
invocation, or time-out, whichever comes first.

Test: run test 200 times
Bug: 69005254
Change-Id: I79a416f9ba5c93d7e8f9f2d6e32a0f46410561d7