Tests can fail because there is leftover data from a previous run.
For example:

--- FAIL: TestRemoveAll (0.00s)
	path_test.go:96: RemoveAll "/private/var/mobile/Containers/Data/Application/66247524-5ED7-45A4-82AA-6BF15D6078B2/tmp//_TestRemoveAll_" (first): open /private/var/mobile/Containers/Data/Application/66247524-5ED7-45A4-82AA-6BF15D6078B2/tmp//_TestRemoveAll_/dir: permission denied
FAIL
FAIL	os	31.275s

There seem to be no way to simply clear the app data for an app
short of uninstalling it, so do that.

This change in effect undoes CL 106676, which means that running iOS
is a little slower again, and that another app from the same
apple developer account must be present on the device for our app
install to succeed.

Change-Id: Iacc3a6f95c93568f4418db45e1098c7c7fdb88e0
Reviewed-on: https://go-review.googlesource.com/111795
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>