compiletest: Clarify that --no-capture is needed with --verbose

Confusingly, this does not make compile test print what command is used
to run a ui test:

    ./x test tests/ui/panics/abort-on-panic.rs --verbose

It is also necessary to pass `-- --no-capture`, like this:

    ./x test tests/ui/panics/abort-on-panic.rs --verbose -- --no-capture

Now you will see prints like this:

    executing cd "/rust/build/x86_64-unknown-linux-gnu/test/ui/panics/abort-on-panic.next" && \
        RUSTC="/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc" \
        RUST_TEST_THREADS="32" \
        "/rust/build/x86_64-unknown-linux-gnu/test/ui/panics/abort-on-panic.next/a"

Add a hint in the code for this that would have helped me figure this out.

Author: Enselic

src/tools/compiletest/src/util.rs

@@ -29,6 +29,7 @@ fn path_div() -> &'static str {
 pub fn logv(config: &Config, s: String) {
     debug!("{}", s);
     if config.verbose {
+        // Note: `./x test ... --verbose -- --no-capture` is needed to see this print.
         println!("{}", s);
     }
 }