[ffx] Handle target infos in parallel.

When resolving device infos, do so in parallel. In the event that a
user-mode emulator was online and we attempted to probe it, other
devices would not be discovered.

This was tested manually with a TAP and a SLiRP target. I terminated the
SLiRP target while preserving the instance directory, then kept the TAP
target online, and noticed that only the SLiRP target was displayed by
`ffx target list`.

This also introduces a regression test.

Test: fx test ffx_list_test
Fixed: 555903414
Change-Id: I4e38305c8a81d9e67a0b789be5b69ad86a6a636c
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1800139
Commit-Queue: Andrew Davies <awdavies@google.com>
Fuchsia-Auto-Submit: Andrew Davies <awdavies@google.com>
Reviewed-by: Steven Grady <slgrady@google.com>