test(core): add unit tests for `merge` trait #6417

Job	Run time
Check no_std	43s
Check WASM	1m 7s
Rust fmt	11s
Build and test (stable, true, --no-default-features --features miniscript/no-std)	1m 42s
prepare	5s
Build and test (stable, true, --all-features)	3m 43s
Build and test (1.63.0, --no-default-features --features miniscript/no-std)	2m 27s
Build and test (1.63.0, --all-features)	5m 14s
Build & Test Examples (example_cli)	41s
Build & Test Examples (example_bitcoind_rpc_polling)	50s
Build & Test Examples (example_electrum)	58s
Build & Test Examples (example_esplora)	58s
Build & Test Examples (example_wallet_electrum)	1m 4s
Build & Test Examples (example_wallet_esplora_async)	1m 20s
Build & Test Examples (example_wallet_esplora_blocking)	1m 2s
Build & Test Examples (example_wallet_rpc)	37s
clippy_check	1m 42s
	24m 24s

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