whitespace: Avoid mutating original buffer's markers in clones

whitespace: Avoid mutating original buffer's markers in clones

* lisp/whitespace.el (whitespace--clone): New hook function that is
run after cloning a buffer that copies `whitespace-bob-marker' and
`whitespace-eob-marker' and changes the copies to point to the new
buffer (Bug#59618).
(whitespace-color-on): Register the hook function.
(whitespace-color-off): Unregister the hook function.

* test/lisp/whitespace-tests.el
(whitespace-tests--with-test-buffer): New macro.
(whitespace-tests--check-markers): New function.
(whitespace-tests--indirect-clone-breaks-base-markers)
(whitespace-tests--indirect-clone-markers)
(whitespace-tests--regular-clone-markers): New tests.