Fix flaky test in AtlasJanusGraphIndexClientTest#testGetTopTermsRandom4

[bee0511]

What changes were proposed in this pull request?

The test AtlasJanusGraphIndexClientTest#testGetTopTermsRandom4 failed intermittently because HashMap iteration order is non-deterministic.
Changed generateTerms(...) to use LinkedHashMap so iteration order is consistent.

This is a test-only change.

How was this patch tested?

• Reproduced flaky failures with NonDex, a tool that systematically perturbs iteration order to expose order-dependent bugs.

• Example command:

  mvn -pl graphdb/janus edu.illinois:nondex-maven-plugin:2.1.7:nondex \
    -Dtest=org.apache.atlas.repository.graphdb.janus.AtlasJanusGraphIndexClientTest#testGetTopTermsRandom4 \
    -Drat.skip

• Before fix:

[INFO] Running org.apache.atlas.repository.graphdb.janus.AtlasJanusGraphIndexClientTest
[ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.806 s <<< FAILURE! - in org.apache.atlas.repository.graphdb.janus.AtlasJanusGraphIndexClientTest
[ERROR] org.apache.atlas.repository.graphdb.janus.AtlasJanusGraphIndexClientTest.testGetTopTermsRandom4  Time elapsed: 0.423 s  <<< FAILURE!
java.lang.AssertionError: expected [8] but found [7]
        at org.apache.atlas.repository.graphdb.janus.AtlasJanusGraphIndexClientTest.assertOrder(AtlasJanusGraphIndexClientTest.java:143)
        at org.apache.atlas.repository.graphdb.janus.AtlasJanusGraphIndexClientTest.testGetTopTermsRandom4(AtlasJanusGraphIndexClientTest.java:99)

• After fix: test passes consistently under repeated NonDex runs.