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Add an apptest meta-target in mojo/BUILD.gn.
Move the apptests targets specified in src/BUILD.gn.
Add the remaining apptest targets (clipboard, VM, WM).

Was limited to release, allow debug, but exclude component.
See gn_debug compile error from including component build in PS2:
  FAILED: /b/build/slave/linux_chromium_gn/build/src/buildtools/linux64/gn --root=/b/build/slave/linux_chromium_gn/build/src -q gen //out/Debug/
  ERROR at //mojo/shell/BUILD.gn:13:1: Assertion failed.
  assert(!is_component_build)

BUG=NONE
TEST=Building mojo:apptests builds all local apptest binaries (on dbg too!).
R=sky@chromium.org
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/1050233005

Cr-Commit-Position: refs/heads/master@{#325570}