[TEST] Make detection of compilation mode more robust

The compilation mode in JTreg is parsed from the "java.vm.info" system property. The current implementation expects that "java.vm.info" contains exactly one of the strings "mixed mode", "compiled mode" or "interpreted mode" otherwise the detection will fail and JTreg will quit with an error.

The detection can be done more robust by searching for the substrings "mixed mode", "compiled mode" or "interpreted mode" thus allowing OpenJDK builders to store additional information in the "java.vm.info" system property.