Resolve all warnings in run-coverage tests

When one of these tests fails, any compiler warnings will be printed to the
console, which makes it harder to track down the actual reason for failure.

(The outstanding warnings were found by temporarily adding `-Dwarnings` to the
compiler arguments for `RunCoverage` in `src/tools/compiletest/src/runtest.rs`.)

tests/run-coverage/unused_mod.coverage

@@ -1,4 +1,5 @@
 $DIR/auxiliary/unused_mod_helper.rs:
+   LL|       |#[allow(dead_code)]
    LL|      0|pub fn never_called_function() {
    LL|      0|    println!("I am never called");
    LL|      0|}