Split application_test_main.cc out of *_base.cc.

Move MojoMain for apptests to a new file.
Add mojo::test accessors for the shell handle.

BUG=392646
TEST="mojo_shell mojo:example_apptests" still works.
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/680263004