tests: clean out more temporary files

tests: clean out more temporary files

What remains is a bunch of anonymous untraceable /tmp/tmp.XXXXXXXXXX
files and bak.root.receive.staff1.3835 from an error branch, testdir.1,
testdir.3, and testroot454470 (with children) in testroot

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13259