Fixes #173 Adds to clean-up the switch theme via WP CLI

[hanna-meda]

Description

Updated clean-up method to include switch to 2020 theme via WP CLI
Removed step that did this in a specific scenario as it is no longer needed there now

Fixes #173
Explain how this code impacts users.

Type of change

• New feature (non-breaking change which adds functionality).
• Bug fix (non-breaking change which fixes an issue).
• Enhancement (non-breaking change which improves an existing functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as before).
• Sub-task of #(issue number)
• Release

Detailed scenario

This can be checked on PR by:

1. Running first this scenario & check theme in UI ===> active theme is Astra
2. Run any other scenario & check theme in UI ===> theme is switched to 2020

Technical description

As per grooming

Documentation

As grooming

New dependencies

List any new dependencies that are required for this change.

Risks

List possible performance & security issues or risks, and explain how they have been mitigated.

Mandatory Checklist

Code validation

• I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Code style

• I wrote a self-explanatory code about what it does.
• I protected entry points against unexpected inputs.
• I did not introduce unnecessary complexity.
• Output messages (errors, notices, logs) are explicit enough for users to understand the issue and are actionnable.

Additional Checks

• In the case of complex code, I wrote comments to explain it.
• When possible, I prepared ways to observe the implemented system (logs, data, etc.).
• I added error handling logic when using functions that could throw errors (HTTP/API request, filesystem, etc.)

[hanna-meda]

Running test:e2e on PR with latest changes, with 3.17.3.zip as new release, only 1 failing test remains, related to "About Us" #170:
cucumber-report-178-e2e.html.zip

Update: with 3.17.3.1.zip, results are different, 5 failing:
cucumber-report-178-3.17.3.1.html.zip