compiletest: show usage/help when passed no args

instead of this panic:
```
thread '<main>' panicked at 'index out of bounds: the len is 1 but the
index is 1', src/libcollections/vec.rs:1110
```

It still panics, just like `-h` does, so it should be okay in this
regard.

src/compiletest/compiletest.rs

@@ -92,7 +92,7 @@ pub fn parse_config(args: Vec<String> ) -> Config {
           optflag("h", "help", "show this message"));
 
     let (argv0, args_) = args.split_first().unwrap();
-    if args[1] == "-h" || args[1] == "--help" {
+    if args.len() == 1 || args[1] == "-h" || args[1] == "--help" {
         let message = format!("Usage: {} [OPTIONS] [TESTNAME...]", argv0);
         println!("{}", getopts::usage(&message, &groups));
         println!("");