Process replay tests

Process replay is a tool for creating a diff of generated kernels between two commits. By default, process replay doesn't assert kernel diffs.

Refactor and speedup PRs must enable the assert by including [pr] in the pull request title.

Running locally

To run process replay locally:

(optional: clear previous process replay runs with test/external/process_replay/reset.py)

Run tests with CAPTURE_PROCESS_REPLAY=1 in your branch. This will pickle process inputs to CACHEDB.
Checkout master
Run test/external/process_replay/process_replay.py

For reference, see test/external/process_replay/local.sh.