gcjob_test: deflake TestSchemaChangeGCJob

[rafiss]

This updates a test assertion so that if the GC TTL already began, the test does not fail.

fixes #117485
fixes #118467
Release note: None

[cockroach-teamcity]

This change is 

[fqazi]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rafiss)

[rafiss]

flake was unrelated (it was TestRegionLivenessProberForLeases #118464). tftr!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from f2afcc1 to blathers/backport-release-23.1-118498: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 23.1.x failed. See errors above.

---

error creating merge commit from f2afcc1 to blathers/backport-release-23.2-118498: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 23.2.x failed. See errors above.

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