Fixes flaky continuous transforms test

[downsrob]

Signed-off-by: Robert Downs downsrob@amazon.com

Issue #, if available:
N/A
Description of changes:

./gradlew ':integTest' --tests "org.opensearch.indexmanagement.transform.TransformRunnerIT.test continuous transform with wildcard indices" -Dtests.seed=7456CA873D2951BB -Dtests.security.manager=false -Dtests.locale=es-US -Dtests.timezone=Etc/Greenwich -Druntime.java=14
org.opensearch.indexmanagement.transform.TransformRunnerIT > test continuous transform with wildcard indices FAILED
    java.lang.NullPointerException
        at __randomizedtesting.SeedInfo.seed([7456CA873D2951BB:9E30041088F9C85C]:0)
        at java.base/java.time.Instant.compareTo(Instant.java:1255)
        at java.base/java.time.Instant.isAfter(Instant.java:1272)
        at org.opensearch.indexmanagement.transform.TransformRunnerIT.test continuous transform with wildcard indices(TransformRunnerIT.kt:771)

Noticed this flaky failure for continuous transforms. Metadata with the transform afterkey set to null was updated before the final step where the ContinuousStats were updated, so a test scenario in which the continuous stats were missing was possible. This change makes the waitFor loop wait for the continuous stats to be present.
CheckList:

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[codecov-commenter]

Codecov Report

Merging #276 (8c0d7ca) into main (97a8405) will decrease coverage by 0.22%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #276      +/-   ##
============================================
- Coverage     76.48%   76.26%   -0.23%     
+ Complexity     2027     2021       -6     
============================================
  Files           262      262              
  Lines         11749    11749              
  Branches       1869     1869              
============================================
- Hits           8986     8960      -26     
- Misses         1741     1767      +26     
  Partials       1022     1022              

Impacted Files	Coverage Δ
...statemanagement/model/destination/CustomWebhook.kt	65.21% <0.00%> (-27.54%)	⬇️
...arch/indexmanagement/rollup/RollupSearchService.kt	55.55% <0.00%> (-3.71%)	⬇️
...dexmanagement/transform/model/TransformMetadata.kt	88.11% <0.00%> (-2.98%)	⬇️
...ndexstatemanagement/IndexStateManagementHistory.kt	77.30% <0.00%> (-2.84%)	⬇️
...ent/indexstatemanagement/util/ManagedIndexUtils.kt	78.48% <0.00%> (-1.27%)	⬇️
.../rollup/action/start/TransportStartRollupAction.kt	69.76% <0.00%> (+2.32%)	⬆️
...nt/indexstatemanagement/model/destination/Slack.kt	54.54% <0.00%> (+13.63%)	⬆️

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