schemachanger: Unskip some backup tests

[Xiang-Gu]

Randomly skip subtests in the BACKUP/RESTORE suites before parallelizing them.

Epic: None
Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[Xiang-Gu]

CI succeeded for 4-5 runs when it's in draft so I think it's ready for a review to unskip those tests.

[ecwall]

Are the parallelization and skipThreshold changes part of deflaking the skipped tests? If so, could you explain how those changes deflake the tests?

[Xiang-Gu]

@ecwall They make those test run less subtests (by getting rid of skipThreshold) and faster (by skipping a test before calling t.Parallel() which otherwise would pause the process and continue it later only to find out it will be skipped anyway).

[Xiang-Gu]

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)