Reduce duplication across test scripts and reuse code across java runtimes tests

[balopat]

• There is quite a bit of duplicated code in the test scripts (https://github.com/GoogleCloudPlatform/openjdk-runtime/blob/master/scripts/).
• There is a deviation from the structure in tomcat-runtime and the jetty-runtime

It seems it would be beneficial to design an overarching testing strategy for our runtimes, instead of duplicating the efforts across them, most probably relying on the runtimes-common infrastructure as much as possible.