Allow specifying binary path in tests

Currently, the path that the tests use to find the diffr binary expects that the tests are being run from cargo:

Lines 59 to 65 in 2c87325

    
           fn diffr_path() -> PathBuf { 
        
               let mut dir = PathBuf::from(env!("CARGO_MANIFEST_DIR")); 
        
               dir.push("target"); 
        
               dir.push("debug"); 
        
               dir.push("diffr"); 
        
               dir 
        
           }

This can cause issues when packaging diffr for distributions (see NixOS/nixpkgs#69505, where doCheck = false disables running the package's tests).

It would be great if there was a way to provide the path, with $CARGO_MANIFEST_DIR/target/debug/diffr being a fallback, perhaps through a DIFFR_TESTS_BINARY_PATH environment variable or something like that.

	fn diffr_path() -> PathBuf {
	let mut dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
	dir.push("target");
	dir.push("debug");
	dir.push("diffr");
	dir
	}