RFR(XS): 8212154: [TESTBUG] CheckArchivedModuleApp fails with NPE when JVMCI is absent

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8212154

webrev: http://cr.openjdk.java.net/~ccheung/8212154/webrev.00/

The ArchivedModuleComboTest.java and the 
ArchivedModuleWithCustomImageTest.java failed with NPE if tested with a 
JDK build without JVMCI. A fix is to add a null check on 
wb.getBooleanVMFlag("EnableJVMCI").

Testing:
     Ran the 2 tests locally on linux-x64 with JDK builds (with and 
without JVMCI).

thanks,
Calvin