[ios] rls_end2end_test fix

[dennycd]

Turns out when bazel run ios unit test, gtest's SetupTestSuite are invoked per unit test case, which would trigger assertion failure on duplicate registry on the 2nd and subsequent test case runs.

Alternative fix for calling ShutdownRegistry in test suite teardown doesn't seem to work as this seems to have reset internal states and causes all test case failures.

Follow up with grpc/grpc-ios#41 for investigating into why iOS is loading / running test this way.

[markdroth]

I don't understand why this change would help. Even if the problem is that gtest is running SetUpTestSuite() per test case instead of once per test suite, then re-registering the LB policy after calling grpc_init() should be fine; it just means that we're doing all gRPC setup once per test case instead of once per test suite. The previous registry of the LB policy should not be present after calling grpc_shutdown_blocking() in TearDownTestSuite().

Why is the LB policy still registered after shutting down gRPC?

[markdroth]

I think I'm going to make this particular problem go away by removing RLS from the gRPC build on mobile, since we've had unrelated complaints about the binary size increase with this policy. But it would be nice to still figure out what the actual root cause is here.

[markdroth]

#27838 attempts to disable RLS on mobile.