[fix][test] Fix flaky ManagedLedgerTest.testNoRetention

[merlimat]

Motivation

ManagedLedgerTest.testNoRetention is flaky because it relies on Thread.sleep(1000) for the async ledger trimming to complete, which is not always sufficient.

Stack trace

ManagedLedgerTest > testNoRetention FAILED
    java.lang.AssertionError: expected [true] but found [false]
        at org.testng.Assert.fail(Assert.java:110)
        at org.testng.Assert.failNotEquals(Assert.java:1577)
        at org.testng.Assert.assertTrue(Assert.java:56)
        at org.testng.Assert.assertTrue(Assert.java:66)
        at ManagedLedgerTest.testNoRetention(ManagedLedgerTest.java:2316)

Modifications

Replace Thread.sleep(1000) with explicit trimConsumedLedgersInBackground(CompletableFuture) + join() to deterministically wait for ledger trimming to complete before asserting.

Documentation

• doc-not-needed