CQ-4355928: add proxy availability check

[bisswanger]

The proxy changes have been added with a previous PR though are slightly incomplete
In case the proxy is not available, the UI test would be started and then most likely fail

Thus adding the proxy availability check, same as on https://github.com/adobe/aem-test-samples/pull/93/files

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.