Fix for issue #2765 relative to unstable trafficshaping test procedure

Motivation:
The test procedure is unstable due to not enough precise timestamping
during the check.

Modifications:
Reducing the test cases and cibling "stable" test ("timestamp-able")
bring more stability to the tests.

Result:
Tests for TrafficShapingHandler seem more stable (whatever using JVM 6,
7 or 8).

Will be ported to 4.1, 3.9 and master