RFR: JDK-8140282 Remove test directories on clean-test-*

[Magnus Ihse Bursie]

Using some makefile magic, we transform clean-test-foo-bar-baz into a 
call to "rm -r $b/support/test/foo/bar/baz".

However, this might leave a chain of empty directories 
$b/support/test/foo/bar, which is not so nice. We should remove these 
directories as well, if there is no other content in them.

Bug: https://bugs.openjdk.java.net/browse/JDK-8140282
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8140282-remove-test-directories/webrev.01

/Magnus