AjcTaskTest: Be more lenient with aspectjrt version warning

AjcTaskTest: Be more lenient with aspectjrt version warning

Filter out a warning which occurs, if the current release does not match
the stored binary in lib/test:

  bad version number found in aspectjrt.jar
  expected 1.9.21.M1 found 1.9.20.1

If e.g. we run tests for a milestone release a.b.5.M1 and afterwards
switch back to a.b.5-SNAPSHOT, we do not want to update lib/test for a
single commit, just to make this test pass. Hence, we ignore this
warning here.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>