ART: Clean up test exec code

Add a helper for fork+exec of another program and collection of
that process' output. Use the helper in other code.

Clean up some tests. Move away from global #ifdef ARCH and
disable tests with the usual-style macros so that it's easier
to see refactoring issues immediately.

Test: mmma
Test: m test-art-host
Change-Id: Ic450e8a3bb24fc6fe423c0e1e007eb0bb34e22b4