Make TestInterceptor thread-safe

Previously, TestInterceptor used an int to keep a count of how often
it had been called. The count was incremented on one thread and
read on another thread. This lead to intermittent test failures as the
field was not declared volatile and a stale value would sometimes be
returned.

This commit updates TestInterceptor to use an AtomicInteger that's
held in a final field. This ensures that getCount() will not return
stale values and also ensures that the count can safely be incremented
concurrently.

Closes gh-1997