8263470: Consolidate copies of getClassBytes in various tests

...ot/jtreg/runtime/BadObjectClass/TestUnloadClassError.java

...ot/jtreg/runtime/ClassUnload/ConstantPoolDependsTest.java

...spot/jtreg/runtime/ClassUnload/DictionaryDependsTest.java

test/hotspot/jtreg/runtime/ClassUnload/KeepAliveClass.java

...tspot/jtreg/runtime/ClassUnload/KeepAliveClassLoader.java

test/hotspot/jtreg/runtime/ClassUnload/KeepAliveObject.java

...pot/jtreg/runtime/ClassUnload/KeepAliveSoftReference.java

.../hotspot/jtreg/runtime/ClassUnload/MyDiffClassLoader.java

test/hotspot/jtreg/runtime/ClassUnload/SuperDependsTest.java

...otspot/jtreg/runtime/ClassUnload/UnloadInterfaceTest.java

test/hotspot/jtreg/runtime/ClassUnload/UnloadTest.java

...hotspot/jtreg/runtime/DefineClass/NullClassBytesTest.java

test/hotspot/jtreg/runtime/MemberName/MemberNameLeak.java

...estmates/membership/TestNestHostErrorWithClassUnload.java

...ot/jtreg/runtime/cds/appcds/customLoader/HelloCustom.java

...treg/runtime/cds/appcds/customLoader/HelloCustom_JFR.java

...cds/appcds/customLoader/UnloadUnregisteredLoaderTest.java

...ime/cds/appcds/customLoader/test-classes/HelloUnload.java

...s/customLoader/test-classes/UnloadUnregisteredLoader.java

...runtime/cds/appcds/dynamicArchive/HelloDynamicCustom.java