Add test overrides for expected FXC_BASE_URL

[janbrasna]

Description

In case FXC_BASE_URL is set to something else than current default branch value, a lot of tests would start disagreeing given the expected assertions.

Steps to reproduce

FXC_BASE_URL=ftp://example.tld pytest bedrock/firefox

Expected result

Overrides added so that tests still use the expected URL and pass.

Actual result

Environment

FXC_BASE_URL=ntp://example.tld

[janbrasna]

With #16443 merged allowing for flexibility in the settings, I'm leaving this open in case there's appetite to actually repurpose #16414 for adding stability into the redirect tests locking them to fixed values.

Instead of loading whatever is in settings and comparing to real current settings, the ideal result would be keeping the target test results still hard-coded to some expected URL, and only reload that value for test purposes; to make sure the tests are not actually changing depending on settings dynamically, but are defined statically in their expected context.