`zig_test` test_runner should accept a zig_library

[gwenzek]

The end goal is to allow zml users to use our async test runner in their project.
Currently test_runner accept a file, but the file is not pulled into the build sandbox.
Also a test_runner.zig may have dependencies (async in our case) so accepting a file is semantically incorrect.

The current workaround consist of declaring test_runner.zig has part of the zml lib ensuring that it is always present in the build sandbox, but it should only appear on when building the test target.