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

[Tim Bell]

Erik:
> 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

Looks good to me.

Tim