[asset-conversion] Fix benchmarks swap_exact_tokens_for_tokens / swap_tokens_for_exact_tokens

[bkontur]

e.g. for AssetHubWestend we use by default pub storage AllowMultiAssetPools: bool = false, so these two benchmarks fails.

problem is in the module: substrate -> frame/asset-conversion/src/benchmarking.rs that these two benchmarks does not count with T::AllowMultiAssetPools settting, in case of T::AllowMultiAssetPools = false benchamark should be able "prepare" valid data.

How to test:

1. cd <substrate-dir>
2. frame/asset-conversion/src/mock.rs change pub storage AllowMultiAssetPools: bool = false;
3. run tests (dont mind other failing stuff, just these two are important for correct benchmarking)

cargo test -p pallet-asset-conversion --features runtime-benchmarks 

Based on branch: paritytech/substrate#14572 or if merged then master