Fix userlock.el and filelock-tests in some rare cases

Fix userlock.el and filelock-tests in some rare cases

* lisp/userlock.el (userlock--check-content-unchanged): Don't
assume 'file-truename' of FILENAME is always non-nil.  It could be
nil if we are called from a temporary buffer where some Lisp
program has forcibly set 'buffer-file-name'.  (Bug#64821)

* test/src/filelock-tests.el (filelock-tests--fixture): Set
'buffer-file-name' to the true name of the temporary file.
Patch by Mattias Engdegård <mattias.engdegard@gmail.com>.
(filelock-tests-detect-external-change): Quote the file names
passed to shell commands.