Add testonly to libraries that link XCTest

[dierksen]

Tested locally to ensure that testonly = True is added only to the appropriate targets.

[thiagohmcruz]

Aren't there any tests for this? If not maybe we can add one?

[luispadron]

Nice we're doing this in our rules currently but this makes more sense.

[jerrymarino]

@justinseanmartin I wonder if can add thse flags differently based on what Maxwell added to rules_swift to get it working with pods

What do you think if we added back the old behavior into rules_ios conditionally - or adding more intricate logic to detect adding it? I'm still not 100% sure the original motivation to gut the flags.

[jerrymarino]

ref: bazelbuild/rules_swift#868 (comment)

[luispadron]

We'll need to update rules_ios since this now passes testonly for unit test targets which causes duplicate args because rules_ios always passes testonly = True for all test targets as well.

00:05:44.389 	File "/private/var/tmp/_bazel_build/5f8e42350ee950cd952d5c32bb6b1f7e/external/build_bazel_rules_ios/rules/test.bzl", line 61, column 28, in _ios_test
00:05:44.389 		library = apple_library(name = name, namespace_is_module_name = False, platforms = {"ios": ios_test_kwargs.get("minimum_os_version")}, testonly = True, **kwargs)
00:05:44.389 	File "/Users/build/.jenkins/workspace/PR+cas+BCVAFG+PR-Bazel-Optional/ios-builder/s/c/Tools/Rules/library.bzl", line 14, column 5

[luispadron]

This should fix the above issue: bazel-ios/rules_ios#609