pytest: add file removal helpers for TestCaseInTempDir

pytest: add file removal helpers for TestCaseInTempDir

In several places we end a test by deleting a number of files and
directories, but we do it rather haphazardly with unintentionally
differing error handling. For example, in some tests we currently have
something like:

        try:
            shutil.rmtree(os.path.join(self.tempdir, "a"))
            os.remove(os.path.join(self.tempdir, "b"))
            shutil.rmtree(os.path.join(self.tempdir, "c"))
        except Exception:
            pass

where if, for example, the removal of "b" fails, the removal of "c" will
not be attempted. That will result in the tearDown method raising an
exception, and we're no better off. If the above code is replaced with

        self.rm_files('b')
        self.rm_dirs('a', 'c')

the failure to remove 'b' will cause a test error, *unless* the failure
was due to a FileNotFoundError (a.k.a. an OSError with errno ENOENT),
in which case we ignore it, as was probably the original intention.

If on the other hand, we have

        self.rm_files('b', must_exist=True)
        self.rm_dirs('a', 'c')

then the FileNotFoundError causes a failure (not an error).

We take a little bit of care to stay within self.tempdir, to protect
test authors who accidentally write something like `self.rm_dirs('/')`.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15191
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15189

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Noel Power <npower@samba.org>
(cherry picked from commit 2359741b2854a8de9d151fe189be80a4bd087ff9)