helpers: adjust mock_snakemake to be callable with different root directory

[FabianHofmann]

With this change the mock_snakemake function can be called with a Snakefile from a different root directory. This allows to mock_snakemake in pypsa-eur while using a Snakefile from a higher level snakemake workflow.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.