Re-enable alternative plans in join fuzzer (facebookincubator#9898)

Summary:
Pull Request resolved: facebookincubator#9898

Alternative join plans (including sort + merge join) were accidentaly
disabled in a recent refactor
(facebookincubator#9045).
Re-enabling them in this PR.

Reviewed By: mbasmanova

Differential Revision: D57696398

fbshipit-source-id: d0264a1fa214f0e1b46ab03b54fa76fd5fb59173

velox/exec/fuzzer/JoinFuzzer.cpp

@@ -980,6 +980,10 @@ void JoinFuzzer::verify(core::JoinType joinType) {
       flatBuildInput,
       outputColumns));
 
+  makeAlternativePlans(defaultPlan.plan, probeInput, buildInput, altPlans);
+  makeAlternativePlans(
+      defaultPlan.plan, flatProbeInput, flatBuildInput, altPlans);
+
   const auto tableScanDir = exec::test::TempDirectoryPath::create();
   addPlansWithTableScan(
       tableScanDir->getPath(),