Improve test signing scenarios in MicroBuild installation templates

1. It appears that [test signing might now be available on Linux](https://devdiv.visualstudio.com/DevDiv/_wiki/wikis/DevDiv.wiki/12267/ESRP-Signing-in-Mac-or-Linux-Pipelines). We should update the templates to support this scenario if it is supported.
2. We should emit an error message and/or fail if someone tries to use test signing on a non-supported platform. The current logic skips the MicroBuild installation in this scenario but still allows for signing via SignTool. This causes SignTool to emit errors when it should not. This is confusing from a UX/dev perspective for someone not familiar with signing via arcade.