Now that the iOS exec wrapper uninstalls any existing test app before
installing a new, looking up the device app path might fail. Before,
the lookup always succeeded (even though the path reported might be
stale).

For the iOS builder.

Change-Id: I5667b6fae15f88745bdee796db219a429a26e203
Reviewed-on: https://go-review.googlesource.com/112075
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>