Add tests - DPL.

Add tests - DPL.

[Issue#] N/A
[Feature] Add DPL tests back to wrt-commons. Build tests conditionally.
[Problem] N/A
[Cause] Unit tests should reside in the same repositories the code they test.
Tests should be built only on explicit request.
[Solution] Add DPL tests to wrt-commons repository. Build tests only
when option WITH_TESTS is set.
[SCMRequest] gbs >= 0.12

[Verification]
REQUIRES: gbs >= 0.12
1. Remove `wrt-extra` RPM package from the device, if present.
2. Make sure `dpl-tests-*` binaries are not present on the device.
3. Build repository using gbs with option `--define "WITH_TESTS OFF"`.
4. Check if subdirectory `tests/` was included in the build process
   (it should NOT be included) and if `dpl-tests-*` are included in
   the RPM package (they should NOT be included).
5. Install RPM package created in step 3.
6. Check if `dpl-tests-*` binaries are present on the device (they should
   NOT be present).
7. Build repository using gbs with option `--define "WITH_TESTS ON"`.
8. Check if subdirectory `tests/` was included in build process (it
   should be included) and if `dpl-tests-*` are included in the RPM
   package (they should be included).
9. Install RPM package created in step 7.
10. Run `dpl-tests-*` test suite.

Change-Id: Ib9a595c407ec9a4ac6c64ae9494d1063e1e4e503