kvserver: TestProcessSplitAfterRightHandSideHasBeenRemoved times out

The test timed out under stressrace [here](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_BazelExtendedCi/20614890?expandBuildDeploymentsSection=false&hideTestsFromDependencies=false&expandBuildTestsSection=true&hideProblemsFromDependencies=false&expandBuildProblemsSection=true&expandBuildChangesSection=true).

Looking at the output, it is not the specific test variation that is being stuck:

```
[06:16:22] : [Run stress tests] panic: test timed out after 7m12s
[06:16:22] : [Run stress tests] running tests:
[06:16:22] : [Run stress tests] TestProcessSplitAfterRightHandSideHasBeenRemoved (7m12s)
[06:16:22] : [Run stress tests] TestProcessSplitAfterRightHandSideHasBeenRemoved/lease-type=LeaseLeader (44s)
[06:16:22] : [Run stress tests] TestProcessSplitAfterRightHandSideHasBeenRemoved/lease-type=LeaseLeader/(1)_no_RHS_partition (44s)
```

Rather, the entire family of `TestProcessSplitAfterRightHandSideHasBeenRemoved` collectively exceed the timeout.

Need to either make the test cheaper, or bump the timeout.

Jira issue: CRDB-55519

Locate TestProcessSplitAfterRightHandSideHasBeenRemoved and run it under stressrace, using the linked timeout output as the starting point. Inspect the test family’s total runtime rather than only the no-RHS partition variation. Done means the test family completes within the configured timeout, either by reducing its cost or by applying a justified timeout increase.

Written by the indexing model from the issue text.