Fix unreliable DeleteShardTest

[pjmcarthur]

https://issues.apache.org/jira/browse/SOLR-16403

Description

#1926 introduced unreliability into the DeleteShardTest by inadvertently removing the wait on observing the new Shard state. This could cause the test to fail depending on timings.

Solution

Correct the issue by using the class method to change the Shard state (as intended), instead of calling ShardTestUtil directly, restoring the wait on the state change being observed.

Tests

This is a test change. The test runs reliably once more.

Checklist

Please review the following and check all that apply:

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• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide