Directory name used by test_merged_config is not rust unique

[davidBar-On]

test_merged_config() in confi.rs creates and uses folder name a, usually under /tmp directory. The problem is that /tmp/a is a commonly used for other purposes, and that can cause problems as happened to me in #4400. It would be better if a more rust unique name will be used instead of "a".

[calebcartwright]

This can be addressed as simply as changing the parent directory name from a to something a bit more unique in:

rustfmt/src/config.rs

Lines 644 to 680 in 334e297

	#[test]
	fn test_merged_config() {
	match option_env!("CFG_RELEASE_CHANNEL") {
	// this test requires nightly
	None | Some("nightly") => {
	let _outer_config = make_temp_file(
	"a/rustfmt.toml",
	r#"
	tab_spaces = 2
	fn_call_width = 50
	ignore = ["b/main.rs", "util.rs"]
	"#,
	);
	let inner_config = make_temp_file(
	"a/b/rustfmt.toml",
	r#"
	version = "two"
	tab_spaces = 3
	ignore = []
	"#,
	);
	let inner_dir = inner_config.path.parent().unwrap();
	let (config, paths) = load_config::<NullOptions>(Some(inner_dir), None).unwrap();
	assert_eq!(config.tab_spaces(), 3);
	assert_eq!(config.fn_call_width(), 50);
	assert_eq!(config.ignore().to_string(), r#"["main.rs"]"#);
	let paths = paths.unwrap();
	assert!(paths[0].ends_with("a/rustfmt.toml"));
	assert!(paths[1].ends_with("a/b/rustfmt.toml"));
	}
	_ => {}
	};
	}