TestMockDirectoryWrapper.testIsLoadedOnSlice failure

### Description

```
TestMockDirectoryWrapper > testIsLoadedOnSlice FAILED
java.lang.AssertionError
at __randomizedtesting.SeedInfo.seed([7AA64BB2BA5EE192:30F94BDEF5E7E3DF]:0)
at org.junit.Assert.fail(Assert.java:87)
at org.junit.Assert.assertTrue(Assert.java:42)
at org.junit.Assert.assertTrue(Assert.java:53)
at org.apache.lucene.tests.store.BaseDirectoryTestCase.testIsLoaded(BaseDirectoryTestCase.java:1660)
at org.apache.lucene.tests.store.BaseDirectoryTestCase.testIsLoadedOnSlice(BaseDirectoryTestCase.java:1627)
```

### Gradle command to reproduce

```
gradlew :lucene:test-framework:test --tests "org.apache.lucene.tests.store.TestMockDirectoryWrapper.testIsLoadedOnSlice" -Ptests.file.encoding=UTF-8 -Ptests.gui=false -Ptests.jvmargs= -Ptests.jvms=12 -Ptests.nightly=true -Ptests.seed=7AA64BB2BA5EE192 -Ptests.vectorsize=128
```

Note for reproduction: `tests.nightly=true` is important here as the random wrapping of Directories uses `rarely()`, which changes its behaviour depending on the `nightly` setting.

Run the listed Gradle command with the provided seed and nightly setting to reproduce TestMockDirectoryWrapper.testIsLoadedOnSlice. Read BaseDirectoryTestCase.testIsLoadedOnSlice and testIsLoaded, then inspect the related test framework code to identify why the assertion fails. Done means the targeted test passes under the supplied configuration.

Written by the indexing model from the issue text.