Update gRPC ObjC test script to support structured test results in kokoro UI

[dennycd]

Update bazel-based gRPC ObjC test to support generating structured test logs that can be rendered by kokoro UI. Specifically, look into existing c++'s bazel test scripts examples

• https://github.com/grpc/grpc/blob/master/tools/internal_ci/macos/grpc_run_bazel_c_cpp_tests.sh
• https://github.com/grpc/grpc/blob/master/tools/internal_ci/macos/grpc_run_bazel_cpp_ios_tests.sh

Prior PR for structured test results at

• Simplify running bazel with structured test results on CI grpc#29353

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@HannahShiSFB @jtattermusch

[jtattermusch]

For the grpc_objc_bazel_test (here), the structured test results are already being produced:

• they are uploaded to resultstore UI (see example)
• they are being upload to biquery for the continuous jobs (so that we have the historical data about test flakiness).

the problem is that the granularity of the objC bazel test targets is currently very coarse (we only have a handful of high-level targets , e.g. //src/objective-c/tests:MacTests or //src/objective-c/tests:UnitTests, which will make our flakiness insight much less useful than they could be.

@dennycd is there a way in which we can increase the granularity of the test targets?