misc/ios: make it easier to run benchmarks on the device

[josharian]

I have some hacks I carry around locally to enable benchmarking on an iOS device:

• Make tests print "DONE" after running benchmarks, similarly to the way they currently print "PASS" after running tests.
• Add an envvar to go_darwin_arm_exec.go to control what string it watches for to detect that a test has completed running, and set that envvar to "DONE" when running benchmarks.
• Add an envvar to go_darwin_arm_exec.go to enable debug mode, rather than it being a constant, and add that envvar when running benchmarks, in order to see incremental output.

This is obviously not a good solution. I don't know what a good solution looks like, but I'd like one. :)

/cc @crawshaw