[BOLT] Set nfc-tests once

When setting up the nfc-mode tests, avoid adding an extra pair of in-tree and
out-of-tree tests that were unconditional. This was previously done with
addNinjaSteps. Instead, add those later and make them conditional on llvm-bolt
being modified.

Since tests are now only added by BOLTBuilder:
- `ninja` runs the in-tree tests to correctly build dependencies.
- flunkOnFailure is set to update the build status on failures.

Some nits:
- the '--switch-back' flag is used (from nfc-check-setup.py).
- a 'nfc-' prefix is appended to 'check-bolt-different' step
- remove timing.log cleanup
1 file changed