schemachanger: deflake TestConcurrentDeclarativeSchemaChanges

[postamar]

This commit deflakes this test by checking that the second schema change actually does block because of the first one, rather than checking that it has blocked. The bug was that the latter wasn't always guaranteed to happen because we didn't force the schema changes to run in parallel.

Fixes #106732.

Release note: None

[cockroach-teamcity]

This change is 

[postamar]

Thanks for the review!

bors r+

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

This PR was included in a batch that timed out, it will be automatically retried

[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 abc0966 to blathers/backport-release-22.2-107636: 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 22.2.x failed. See errors above.

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error creating merge commit from abc0966 to blathers/backport-release-23.1-107636: 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.

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