Warnings when compiling for stable

[dtolnay]

$ cargo check --features stable
   Compiling compiletest_rs v0.3.15
warning: method is never used: `print_source`
   --> src/runtest.rs:349:5
    |
349 |     fn print_source(&self, src: String, pretty_type: &str) -> ProcRes {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: #[warn(dead_code)] on by default

warning: method is never used: `compare_source`
   --> src/runtest.rs:367:5
    |
367 | /     fn compare_source(&self,
368 | |                       expected: &str,
369 | |                       actual: &str) {
370 | |         if expected != actual {
...   |
384 | |         }
385 | |     }
    | |_____^

warning: method is never used: `typecheck_source`
   --> src/runtest.rs:387:5
    |
387 |     fn typecheck_source(&self, src: String) -> ProcRes {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^