misc/ios: retry lldb launch if the iOS app is busy

Sometimes, a newly installed the test app is not ready to launch
or the reported app path is stale. Pause and retry the launch if
the lldb script did not run the program.

Change-Id: Ic7745d4b5a02f2e3cb8134341859039812f65a65
Reviewed-on: https://go-review.googlesource.com/111216


Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>