tests: remove nearly all remaining backticks

tests: remove nearly all remaining backticks

* tests/rm/isatty: Remove now-unneeded "# `" line.
* tests/misc/ls-time: Likewise.
* tests/misc/date-sec: Remove stray "`" in comment.
* tests/du/long-sloop: Likewise.
* tests/mv/part-symlink: Replace multi-line `...` by $(...).
* tests/mv/sticky-to-xpart: Likewise.
* tests/rm/fail-2eperm: Likewise.
* tests/ls/nameless-uid: Likewise.
* tests/cp/perm: Likewise.
* tests/cp/preserve-gid: Likewise.
* tests/cp/same-file: Likewise.