Add a test case for List.dropAt at index in the middle of list #1985

Job	Run time
check-changes	22s
ran-none	0s
start-nix-linux-aarch64-build-default-test / nix-linux-arm64	35m 48s
start-nix-linux-aarch64-cargo-build-test / nix-linux-arm64	43m 19s
start-macos-x86-64-tests / test-rust-macos-x86-64	32m 12s
start-nix-linux-x86-64-tests / nix-linux-x86	38m 9s
start-nix-macos-apple-silicon-tests / nix-apple-silicon	43m 33s
start-roc-benchmarks / benchmark roc programs	10m 50s
start-ubuntu-x86-64-nix-tests-debug / cargo test debug nix	16m 52s
start-ubuntu-x86-64-tests / test zig, rust, wasm...	28m 16s
start-windows-release-build-test / windows-release-build	12m 32s
start-windows-tests / windows-test-subset	24m 9s
ran-full	1s
finish	1s
	4h 46m 4s

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