Update documentation on disabling tests

[dgitis]

With the two sets of user tables, I find myself running in to issues where I want to disable a package model but am dissuaded from doing so because it will cause package tests to fail.

However, I discovered that we can disable tests as well as models.

This works:

unit_tests:
  ga4:
    staging:
      stg_ga4__users:
        +enabled: false

@adamribaudo, we discussed other solutions for this problem a couple of weeks ago but we should use this it is dbt-native.