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).