Google Firebase Testlab

I want to use Firebase Testlab for iOS XCTest. However there are no rules to generate the .xctestrun file. I tried uploading the test .zip file

```
bazel build --ios_multi_cpus=armv7,arm64 //path/to:test
bazel-out/applebin_ios-ios_armv7-fastbuild-ST-d38ae4fb46e6/path/to/test.zip
```

```
Upload invalid: Uploaded zip files must contain a single .xctestrun file and the contents of the DerivedData/Build/Products directory.
```

The .zip file generated using `ios_unit_test` contains a folder .xctest with a binary to run the test. There seems to be no options to generate a compatible .xctestrun which is the XCTest output file by XCode -> Build for Testing.

`bazel test` passed completely but building for `--ios_multi_cpus=x86_64` devices. I have found no documentation for running the connected device for testing also. Should this be a separated issue I will create another issue.

Start by reproducing the `bazel build --ios_multi_cpus=armv7,arm64` output from `ios_unit_test` and compare its zip contents with Firebase Testlab's required `.xctestrun` and `DerivedData/Build/Products` layout. Review the existing `ios_unit_test` behavior and the `bazel test` path; done means producing an upload-compatible archive or documenting that limitation and the connected-device testing workflow.

Written by the indexing model from the issue text.