Declare test toolchain requirements on the `test` exec group

[fmeum]

This ensures that unit and analysis tests can be executed on all platforms, regardless of the current target platform, even after https://github.com/bazelbuild/bazel/issues/25823.

[fmeum]

@gregestren Could you review this?

[gregestren]

LGTM but I don't have codeowner status on this repo. I think @tetromino is a good contact?

[fmeum]

@tetromino @gregestren Friendly ping