`+[SLTest testNamed:]` now allows the user to retrieve focused tests
without specifying the focus prefix.
Fix boolean checks in `subliminal-test`.

$ENABLE will evaluate to true if ENABLE is not set.
Correct the scope of a shell variable.

`SCRIPT_DIR` is actually used below the simulator configuration block.
It just happened that its use below coincided with the execution of this block,
so the variable would be defined when needed.
`subliminal-test` now requires that the user specify a simulator version.

If a version is not specified, it is not clear which version of the simulator to reset.
We also can't guarantee the version of the simulator that will be used unless we set it,
given the variances in the behavior of `instruments` as described in
http://openradar.appspot.com/radar?id=3107401.

It also makes the README a bit simpler to only describe behavior that we can guarantee.
Merge the various simulator scripts into `simulator-set`.

To eliminate redundant logic and simulator restarts, and simplify the CI setup.
Remove `enable_GUI_scripting.scpt`.

Its logic has been incorporated into `authorize_instruments.scpt`,
which was the only script which needed it anymore.
`subliminal-instrument` wraps the invocation of `instruments` and formats its output.

Using ANSI colors and unicode symbols, in the style of xctool, to produce pretty text output.

WIP, so [skip ci]