Harden performance tests for Bug 543480 in CI runs: run GC first, draw
10 samples, ignore the 2 lowest and 2 highest compile times as they are
potential outliers. Further details added to error message.

Change-Id: I0fc091d9802ddf95326dc58abb034721a47592c9
Signed-off-by: Sebastian Lohmeier <sebastian@monochromata.de>