Adding test case type RUNNER_MULTIPROCESS_TEST.

Adding test case type RUNNER_MULTIPROCESS_TEST.

Running many processes in one test case would work fine on those rules.
* Using wait(), waitpid(), waitid() functions within test case body is forbidden
(waiting for child processes is handeled by framework).
* Process exiting properly should exit only 0.
* Communication by signals is forbidden - signals could be cought by parent and
  ignored.

Tests also provided. To run them build this flags below and run them as
wrt-commons-tests-test --output=text --runignored .
  flags:  --define "WITH_TESTS ON" --define "WITH_CHILD ON"
All tests from DPL_TESTS_TEST_MULTIPROCESS group should pass.

[Issue#]        SSDWSSP-438
[Bug/Feature]   Calling fork() inside RUNNER_TEST causes errors for whole process
                group while failed or ignored. Additional path for each redundant
                failed process is created.
[Cause]         N/A
[Solution]      Creating test case type controlling processes.
[Verification]  Applying it to failing multiprocess tests. All errors should
                be collected as one and no additional process should run after
                test case ends.

Change-Id: I371528de102def369107cab3c5cffe0b2f53e9e5