displayed run's estimated duration in api

The javadoc of MavenReporter states clearly that preBuild/postBuild is
only once per the entire build, but the previous code was calling it at
the same timing as enter/leaveModule.

This is now fixed.

Also removed a duplicate constant

JENKINS-13377 test suite duration may be calculated wrong

[Re-FIXED JENKINS-12457] 'Age' column on 'Test Result' tab may show incorrect value when a test suite divided into multiple junit files

Added testcase and changelog for JENKINS-13214. The actual fix is in 7fba6527