Allow soft references in FlowExecutionListTest leak assertions

[RanjitCS]

An upcoming fix to MemoryAssert.assertGC in jenkins-test-harness (jenkinsci/jenkins-test-harness#1029)) will cause assertGC(ref, false) to properly detect soft references. Without this change, these two tests would start failing once that fix lands

The soft references detected are from java.beans.Introspector and Groovy MetaClass invocation caches which is standard JVM/Groovy caching unrelated to the StepExecutionIterator code under test. These tests still verify there are no strong reference leaks

Testing done

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