Add -U option to installer (and make use of it in audio test)

4 commits:
 1. Installer: add a `-U` flag in installer, similar to `-u`, but does not update existing targets.
 2. Do not require `core` in -T tests (so we can replace some `sh -exc` with `crouton -T`)
 3. Some meta-test, to check that -U works as intended (checking of the output is manual, and I prefixed it with `a0` so it does not get pulled during general tests)
 4. Make use of -U in `30-audio`

Tested in `2013-12-10_13-27-04_drinkcat_chroagh_fix_main.sh_T_-R_precise_30_a0`