Gyp fixes for mojo/edk

This fixes up the gyp build for mojo/edk. The new gyp file
mojo/edk/mojo_edk_test.gyps contains unit test targets and is needed to
avoid gyp files cycles between mojo_edk.gyp and mojo_base.gyp. Test
support libraries stay in mojo/edk/mojo_edk.gyp.

Chromium tryjobs here: https://codereview.chromium.org/718193002/
Initial work done by aa@ here: https://codereview.chromium.org/717663002/

R=hansmuller@chromium.org

Review URL: https://codereview.chromium.org/724533002