CFStream Test Suite Bazel Migration

[dennycd]

Our CFStream test suite currently have a few issues that need to be improved

• Test hangs and keep retries until timeout if test failure
• Async test logic is using NSRunLoop mainRunLoop instead of XCTestExpectation

In addition, the current test suite is an objc translation from existing c-core test

• https://github.com/grpc/grpc/blob/master/test/core/iomgr/BUILD#L241

We should revisit the test setup to run c-core's test setup using the corresponding ios unit test target instead

• https://github.com/grpc/grpc/blob/master/bazel/grpc_build_system.bzl#L335

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Additional todos for CFStream Test Update

• Migrate CFStream test to bazel based
• Move CFStream test out of test/core/iomgr and move into src/objective-c so that it is co-located with the rest of the ios/objc test.

[jtattermusch]

Note the the ios-test-cfstream-tests test suite is last one in the
grpc_basictests_objc_ios job.
After it gets migrated to bazel, we can basically get rid of the grpc_basictests_objc_ios job.