Allow JRE testing for other testsuite aside from JCK

So basically what the problem is, is that we are unable to run JRE testing with anything but JCK, since JAVA and JAVAC command variables are only defined and propagated from the jck.mk...
if we want to expand JRE testing on other testsuites we should find a suitable place to move this declarations to.
This also targets the issue of headless testsuite https://github.com/adoptium/aqa-tests/issues/5224 , that was designed with jre testing in mind and is circumventing this in its own way. We should align this with the mechanics used elsewhere in Adoptium pipeline, in this case use the same mechanic for jre testing as used in JCK.