RFR: JDK-8217733 Minor RunTest fixes

[Magnus Ihse Bursie]

This patch covers some different minor cleanups that I accumulated after 
working on RunTest:
 * Do not override JDK_IMAGE_DIR for jcov testing; instead introduce 
JDK_UNDER_TEST (to be really explicit about which JDK we're testing).
 * Remove unconditional $(info) printout on Windows
 * Group JCov variable setup with other variable setups.
Bug: https://bugs.openjdk.java.net/browse/JDK-8217733
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8217733-minor-RunTest-fixes/webrev.01

/Magnus