RFR: 8066808: langtools/test/Makefile should not use OS-specific jtreg binary

[Jonathan Gibbons]

For historical reasons, jtreg used to ship with three identical copies 
of the same launch script, in $JTREG_HOME/{win32,solaris,linux}/bin.

That is finally being phased out in favor of a single script in the 
$JTREG_HOME/bin directory.

This fix updates the langtools/test/Makefile accordingly.

http://cr.openjdk.java.net/~jjg/8066808/webrev.00/

-- Jon