Bazel Migration for gRPC ObjC Interop Test

[dennycd]

Migrate existing Cocoapod based Interop test suite to bazel

• https://github.com/grpc/grpc/tree/master/src/objective-c/tests/InteropTests

Existing pod test target defined in

• https://github.com/grpc/grpc/blob/2badafbc4d246d5165546a61eeb36aa017987d30/src/objective-c/tests/Podfile#L36

Its corresponding kokoro run component at

• https://github.com/grpc/grpc/blob/1cd6e69347cbf62a012477fe184ee6fa8f25d32c/tools/run_tests/run_tests.py#L1075-L1080

Also we should enable GRPC_VERBOSITY=debug environment variable when running these tests

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cc @jtattermusch

[jtattermusch]

The //src/objective-c/tests:InteropTests bazel target already exists (not sure how closely it matches the pod test), but its build seems broken. Once the targets builds & runs reliably, it can be added to the kokoro test jobs being added here: grpc/grpc#29599

[jtattermusch]

Looks like the //src/objective-c/tests:InteropTests builds now, but in my few attempts on the CI, I got the impression that it might be flaky. Let's doublecheck it's flakiness level and enable as part of grpc_objc_bazel_test once possible.

[jtattermusch]

@dennycd should we close this?