tests/util: Improve UChild. Cleanup redundant functions in util.rs.

A short summary of changes:

* Add some basic tests for UChild and the run methods.
* Try more often in a fixed interval to create the tempfile for CapturedOutput.
* Fix drop order of struct fields for better cleanup of temporary files/dirs.
* Mark UChild::wait_with_output and UChild::pipe_in_and_wait_with_output deprecated
* Make CapturedOutput private
* Panic in stdout_all, stdout_all_bytes etc. if output is not captured.
* Rename some methods, refactor, clean up, fix documentation, add try_... methods

tests/by-util/test_yes.rs

@@ -21,6 +21,8 @@ fn run(args: &[&str], expected: &[u8]) {
     let mut child = cmd.args(args).set_stdout(Stdio::piped()).run_no_wait();
     let buf = child.stdout_exact_bytes(expected.len());
     child.close_stdout();
+
+    #[allow(deprecated)]
     check_termination(&child.wait_with_output().unwrap().status);
     assert_eq!(buf.as_slice(), expected);
 }