Add client-side support for custom tests
Support --local and --quickstart testing

Controlled by the (new and optional) "type" field in the instance
configuration, which would be "local" or "quickstart".  Such an instance
bypasses the "max-instances" setting (IOW, can run in parallel with any
number of other instances.)  Snapshot selection is also bypassed, since
there is no VM to restore a snapshot in.

If --local fails due to there being no tests, it reports "not supported"
instead of failing.
Detect whether Apache 2.4 is supported on "ubuntu1310" instances
Don't check out v8-jsshell submodule when not testing extensions
Run with "-m testing" instead of "-m testing.main"
Detect and ignore ZeroDivisionError failures

Happens randomly on install when calculating password hashing rounds if
the VM is fast (or unlucky) enough.