$CLASSPATH << 'some/relative/path' does not obey the Dir.pwd

for example this line https://github.com/jruby/jruby/blob/master/test/jruby/test_higher_javasupport.rb#L694
expect the "java current directory" to match Dir.pwd to work

just run the test_higher_javasupport.rb from different directory then jruby.home and it fails (see #2215 on how to run a test from a different directory and the $LOAD_PATH issue is very similar)
