[Bugfix] Fix TieredSpilloverCache flaky tests

[peteralfonsi]

Description

Fixes flaky tests in TieredSpilloverCacheStatsIT.java (and potentially flaky tests in CacheStatsAPIIndicesRequestCacheIT.java).

Previously index refreshes could happen mid-test, which invalidated the cache and caused the number of hits, misses, etc, to be incorrect. This caused flakiness, and I confirmed setting the refresh interval to a very small value like 50 ms caused more frequent flakiness. I set the index refresh interval to -1 to prevent this from happening, and confirmed there were no longer failures when running each test for 1k iterations.

To avoid future issues, also perform a force merge after index creation, so that merges cannot happen mid-test either.

Related Issues

Resolves #14169, #14281, and #14029.

Check List

• Functionality includes testing.
  - [N/A] API changes companion pull request created, if applicable.
  - [N/A] Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 150c574: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[peteralfonsi]

Flaky tests:
#5426
#12407

[github-actions]

❌ Gradle check result for 5cb11af: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for 4d7d64d: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for 3e5a124: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[peteralfonsi]

Flaky test:
#12407

[github-actions]

❕ Gradle check result for 9c7a031: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 71.74%. Comparing base (b15cb0c) to head (9c7a031).
Report is 430 commits behind head on main.

Files	Patch %	Lines
...search/common/cache/stats/ImmutableCacheStats.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #14333      +/-   ##
============================================
+ Coverage     71.42%   71.74%   +0.32%     
- Complexity    59978    62084    +2106     
============================================
  Files          4985     5117     +132     
  Lines        282275   291713    +9438     
  Branches      40946    42168    +1222     
============================================
+ Hits         201603   209299    +7696     
- Misses        63999    65129    +1130     
- Partials      16673    17285     +612     

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