[BOLT] Mark build skipped when tests did not run

[paschalis-mpeis]

If in-tree tests do not run, mark the whole build as skipped. Note that when this happens, out-of-tree tests also do not run.

This helps when navigating Buildbot's build requests:

• Skipped builds indicate the change was unrelated to BOLT.
• Any other errors are still reported.
• For breaking changes that need reversal, builds will show as SKIPPED or FAILURE depending on whether llvm-bolt was affected, instead of SUCCESS or FAILURE, which can be confusing.

[paschalis-mpeis]

This PR aims to make navigating completed build requests more efficient, and less confusing. Note that the WARNINGS won't be displayed once these patches are merged (see discourse post).

An updated table with example patches after this feature:

#	PATCH	CODE CHANGE	In-Tree	Out-Of-Tree	Build Status
1	e24d86	Modifies only a BOLT test.
2	6090c0	Modifies only a BOLT test.
3	b037d0	Modifies bolt heatmap docs.
4	7a688c	Modifies LLVM: an unsupported backend (ARM).
5	3a9893	Modifies LLVM: a supported backend (AArch64).
6	SHAs that fulfill next column →	when nfc-check-setup was somehow broken	(unconditionally)	(unconditionally)	or if more serious
7	SHAs before both #463 and #142410 get merged	when nfc-check-validation is failing (ie unique ids embedded in binaries)	(unconditionally, as it thinks binaries are different. comparison is flawed though)	(unconditionally, as it thinks binaries are different. comparison is flawed though)	or if more serious

[aaupov]

Not an expert in buildbot infra, looks good overall. Approving to unblock. Please wait a couple of days for other inputs before merging.
cc @gkistanova