RFR 8240704: CheckHandles.java failed "AssertionError: Handle use increased by more than 10 percent."

[Roger Riggs]

Please review a test improvement to avoid counting unexpected handles.
The test for 8239893[1]failed when non-test activities used handles.

The test is modified to do a warmup phase spawning some processes and 
cleaning up.
It then runs the test to create processes looking for no significant 
increase in handles.
The execution mode is limited to the interpreter to avoid potential side 
effects of compilation.
The error condition was corrected to look only for increases in the 
number of handles.
Previously, it looked for a 10% delta between the minimum and maximum 
count of handles.

Webrev:
     http://cr.openjdk.java.net/~rriggs/webrev-handlecount-8240704/

Issue:
   https://bugs.openjdk.java.net/browse/JDK-8240704

Thanks, Roger

[1] 8239893 <https://bugs.openjdk.java.net/browse/JDK-8239893> Windows 
handle Leak when starting processes using ProcessBuilder