sauron: Refactor tests to reduce code repetition

[ca-ruz]

Currently, there are four separate test files for Sauron, each corresponding to one of the four different backends. These test files contain repetitive setup code and assertions, with almost identical logic across all files. This duplication makes the tests harder to maintain, less efficient, and prone to inconsistencies when updates are needed.

We should:
-Consolidate the similar tests into a single parameterized test file.
-Use parameterization to test all four backends within the same structure, reducing redundancy and improving maintainability.

Related to #584.

[chrisguida]

This is needed more than ever to fix the way we use the TEST_NETWORK global variable. See this PR: #649 (comment)

What we really need is a single abstract override fixture (probably in conftest.py) to do all our generic settings (like removing the bcli-related options), then a concrete class that overrides that, one for each network (ie bitcoin, testnet, signet, and mutinynet). That way we can avoid using the TEST_NETWORK global variable entirely.