[IOS] Move ios/testing/wait_util.* to base/test/ios/wait_util.* (cleanup)

Cleans up //ios/testing:ios_test_support that had been kept around to avoid
breaking ios downstream

Bug: 784735
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ib07bf8aa8d5e70a1980c2517fb3dbe709467c5a5
Reviewed-on: https://chromium-review.googlesource.com/1136903
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576937}
4 files changed