RFR(S) : 8164035 : compiler/profiling/spectrapredefineclass_classloaders/Launcher.java failing with Agent JAR not found or no Agent-Class attribute

[Igor Ignatyev]

http://cr.openjdk.java.net/~iignatyev/8164035/webrev.00/
> 57 lines changed: 25 ins; 1 del; 31 mod;

Hi all,

there are two issues w/ compiler/profiling/spectrapredefineclass* tests:
 - spectrapredefineclass_classloaders tries to create a jar file using Agent from compiler/profiling/spectrapredefineclass. 
 - spectrapredefineclass builds compiler.profiling.spectrapredefineclass_classloaders.Agent instead of compiler.profiling.spectrapredefineclass.Agent

the changeset fixes both issues, extracts several string constants and refactors tests to use testlibrary to spawn jar processes. 

webrev: http://cr.openjdk.java.net/~iignatyev/8164035/webrev.00/
jbs: https://bugs.openjdk.java.net/browse/JDK-8164035
testing: changed tests locally on mac and remotely on all supported platforms (in progress)

Thanks,
— Igor