Use constants for repeated test values

[mantricjavier]

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Improvement description

The test uses magic numbers like 6u8 for decimals repeatedly. Consider extracting these into named constants.

const DEFAULT_DECIMAL_PLACES: u8 = 6;
const DEFAULT_POOL_AMOUNT: u128 = 1_000_000;
const DEFAULT_SWAP_AMOUNT: u128 = 1_000;

// Usage:
vec![DEFAULT_DECIMAL_PLACES, DEFAULT_DECIMAL_PLACES]

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