Move several test targets out of the Mojo public SDK.

The test targets currently in //mojo/public depend on the EDK to run and thus
are not part of the public SDK. This CL converts several test targets in the
public SDK into testonly mojo_sdk_source_sets, and adds new test targets under
the EDK that depend on the corresponding source_sets as well as the EDK
mechanism for running the unittests.

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=jamesr@chromium.org

Review URL: https://codereview.chromium.org/791533003