Short desc:
  plugin securitiSignedJars tests sometimes show mixed code warning, but there is no
  visible changes in execution env.
  Java CPL Security mixed code option "Enable - show warning if needed" is always used.

  The questions are:
     - what behavior is correct for these tests: mixed code warning or NO mixed code warning
     - why we can see different behavior for the same tests

Set P3 because the true nature of this issue isn't clear now and the issue can't reproduced on demand, it sometimes happens during automated execution.
It happened more often with PIT builds.

See comments section for details.