There are two important things here:
* We only want to run "jdk-test" if we were running "test" for everything else.
This gets around a bug where jdk-test fails for cross-compile builds (where JNI is involved)
* We can specify a different test target by setting the "test" environment variable.
This is useful for cross-compiling the tests in a docker image
(setting the test_target to "build-test")
The intent of this target is to run our test suite against the installed jre.
This should help prevent our VM from diverging in implementation from the jdk.
The remainder of this commit fixes the problems that this exposes.
