Remove Tramp temp files if advised during tests

Remove Tramp temp files if advised during tests

* lisp/net/tramp-fuse.el (tramp-fuse-name-prefix): New defconst.
(tramp-fuse-mount-point): Use it.

* test/lisp/net/tramp-tests.el (tramp-test-name-prefix): New defconst.
(tramp--test-make-temp-name, tramp-test40-make-nearby-temp-file)
(tramp-test47-read-password, tramp-test47-read-otp-password):
Use it.
(tramp--test-enabled-checked): Move down.
(tramp--test-enabled): Delete all Tramp temp files when
environment variable $TRAMP_TEST_CLEANUP_TEMP_FILES is set.
(tramp-test02-file-name-dissect-separate):
Adapt `tramp-crypt-directories' according to syntax.
(tramp-test47-read-password):
Let-bind `tramp-connection-properties' instead of modifying
`tramp-methods'.

(cherry picked from commit 310ce93d02c5317be589803fbde96fd20b96e496)