RFR: JDK-8184075: Make run-test-prebuilt profile more robust

[Erik Joelsson]

Hello,

The special Jib profile run-test-prebuilt is used when running tests 
through Mach 5. This profile is now used as the base for other profiles 
which requires that the profile always has a target platform configured. 
It should be the build platform by default, but if they testedProfile 
input variable is set, it should be the target platform of that profile.

Bug: https://bugs.openjdk.java.net/browse/JDK-8184075

Webrev: http://cr.openjdk.java.net/~erikj/8184075

/Erik