misc/cgo/test: set PWD when executing 'go test' in an alternate GOPATH

misc/cgo/test: set PWD when executing 'go test' in an alternate GOPATH

That makes the test more friendly to the Android exec script, since it
won't have to evaluate symlinks to find the directory.

Change-Id: I06aae3224d489eed6d7fac7e462361f3bf1dd3da
Reviewed-on: https://go-review.googlesource.com/c/164624
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <mail@eliasnaur.com>