misc/cgo: skip cgotest.TestCrossPackageTests on iOS and set PWD

misc/cgo: skip cgotest.TestCrossPackageTests on iOS and set PWD

I hope that this will fix the tests on iOS, but 'gomote create' isn't
giving me an instance I can test with. (Please patch and test before
approving.)

Updates #15919
Updates #30228

Change-Id: I1b7cd30d5b127a1ad3243b329fa005d229f69a24
Reviewed-on: https://go-review.googlesource.com/c/163726
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <mail@eliasnaur.com>