Refactored equalWithinVariance to use it with any custom variance

[UlyanaAndrukhiv]

Context

This PR refactors the equalWithinVariance test helper to support any custom variance and improves how variance comparisons are used throughout the code. It decouples the function from hard‑coded assumptions, making tests and logic that rely on approximate equality clearer and more flexible.

Tests were updated to use the new custom variance.

[holyfuchs]

A few test files still use inline variance logic instead of equalWithinVariance

• rebalance_undercollateralised_test.cdc (lines 96, 101)
• rebalance_overcollateralised_test.cdc (lines 90, 94)
• position_lifecycle_happy_test.cdc (lines 67–68)
• auto_borrow_behavior_test.cdc (lines 78, 88)
• interest_curve_advanced_test.cdc (lines 195–196, 233–234, 268–269, 431–435, 660–664)

Would be good to convert these to use equalWithinVariance for consistency.

[UlyanaAndrukhiv]

@holyfuchs, I’ve updated the remaining test files for consistency