Thread-local failpoints

[brson]

Is your feature request related to a problem? Please describe.

Failpoint unit tests require taking a global lock, preventing test parallelism. An alternate or complimentary solution to a global lock (#23) would be to have a thread-local failpoint configuration, protected by a guard.

Describe the solution you'd like
Add a thread-local configuration that is protected by a guard that performs teardown.

Describe alternatives you've considered
Global locks: #23

Additional context
This would work for single-threaded test cases, but not generally for tests that require multiple threads.

[BusyJay]

Or the library can expose the FailPointRegistry to users, so users can control the lock pattern more precisely.