RFR 8043572: demo/jvmti/mtrace/TraceJFrame.java fails with AWTError: Can't connect to X11 window server using '$DISPLAY_SITE' as the value of the DISPLAY variable.

[Jaroslav Bachorik]

Please, review a simple test fix.

Issue : https://bugs.openjdk.java.net/browse/JDK-8043572
Webrev: http://cr.openjdk.java.net/~jbachorik/8043572/webrev.00
The reason of failure is similar to JDK-8039432. Test tries to access 
the graphics subsystem unconditionally and on headless system it fails. 
The solution is to include the check for the presence of the graphics 
subsystem.

The patch also contains slight changes to the failure message wording in 
TraceJFrame to make it distinguishable from a failure occurring in 
JFrameCreateTime.

Thanks,

-JB-