RFR : exclude tests from jacoco coverage report

[Igor Ignatyev]

Hi all,

please review the tiny fix.

problem:
tests are excluded from jacoco instrumentation, but they are in reports. 
that causes incorrect overall percentages.

fix:
exclude jtt and all test directories from a report.

http://cr.openjdk.java.net/~iignatyev/graal/jacocoreport_exclude_test/webrev.00/
-- 
Igor