RFR: 8066807: langtools/test/Makefile should use -agentvm not -samevm

[Jonathan Gibbons]

Simple fix to have langtools/test/Makefile use agentvm mode instead of 
samevm mode as the default.  agentvm mode is generally recommended over 
samevm in a batch execution environment because of better recovery after 
test failures.

(Also fixed a couple of archaic default paths that are typically never 
used.)

-- Jon