Move mojo_public_application_unittests out of //mojo/public

The test targets currently in //mojo/public depend on the EDK to run and thus
are not part of the public SDK. On the other hand, it is nice to have the test
code living near the code that it is testing. This CL provides a pattern for
resolving this tension by converting the test targets into testonly
mojo_sdk_source_sets, and adding new test targets under the EDK that depend on
the corresponding source_sets as well as the EDK mechanism for running the
unittests.

This CL implements the change for mojo_public_application_unittests. A followup
CL will implement the change for the other 5 test targets under //mojo/public.
Note that the dependencies from the test targets to the source_sets are
specified via relative paths as part of the work toward having the EDK be
flexible wrt its location in a client repo.

BUG=439082
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/779253002