Add test.toolchain field

[qryxip]

Enables running cargo compete test with old Rust without Rustup Override.

Since rust-analyzer no longer works under Rust 1.42 environment, we have to explicity specify +1.42.0 for testing.

[lnicola]

Hello from rust-analyzer! We might still revert the change that prompted this, but this PR seems like a good idea anyway.

My suggestion here would be to set a different target directory (CARGO_TARGET_DIR) for cargo-compete. If you use the same one, rust-analyzer's check on save and cargo-compete will block each other and overwrite their build artifacts.

[qryxip]

@lnicola Thank you for pointing it out. I will think about it.