release-23.1: streamingccl: deflake TestRandomClientGeneration

[msbutler]

Backport 1/1 commits from #100813.

/cc @cockroachdb/release

---

This patch fixes 4 bugs in TestRandomClientGeneration that were responsible for the persistent flakiness and lack of coverage in this test:

• the randomeStreamClient no longer instantiates keys with a table prefix that collides with the job info table prefix. This collision was the original cause of the flakes reported in ccl/streamingccl/streamingest: TestRandomClientGeneration failed #99343.
• getPartitionSpanToTableId() now generates a correct map from source partition key space to table Id. Previously, the key spans in the map didn't contain keys that mapped to anything logical in the cockroach key space.
• assertKVs() now checks for keys in the destination tenant keyspace.
• assertKVs() now actually asserts that kvs were found. Before, the assertion could pass if no keys were actually checked, which has been happening for months and allowed the bugs above to infest this test.

Fixes #99343

Release note: None

Release justification: test only change

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is