tests/tail: Add all additional tests written during refactoring tail …

…from tip of refactoring branch

Other changes summmary:
* Use module level imports instead of fully qualified imports where appropriate
* Fix intermittent failing test_positive_zero_bytes because of broken pipe
* Cleanup old disabled test for warning of following indefinitely

src/uu/tail/src/text.rs

@@ -21,3 +21,4 @@ pub static BACKEND: &str = "ReadDirectoryChanges";
 pub static FD0: &str = "/dev/fd/0";
 pub static IS_A_DIRECTORY: &str = "Is a directory";
 pub static DEV_TTY: &str = "/dev/tty";
+pub static DEV_PTMX: &str = "/dev/ptmx";