Implement `shell-command-dont-erase-buffer' in Tramp. (Bug#39067)

Implement `shell-command-dont-erase-buffer' in Tramp.  (Bug#39067)

* lisp/net/tramp.el (tramp-handle-shell-command):
Handle `shell-command-dont-erase-buffer'.  (Bug#39067)

* test/lisp/net/tramp-tests.el (shell-command-dont-erase-buffer):
Declare.
(tramp-test10-write-region, tramp-test21-file-links): Use function
symbols.
(tramp--test-async-shell-command): Don't assume that
`async-shell-command' returns the process object.
(tramp-test32-shell-command): Rework `async-shell-command-width' test.
(tramp-test32-shell-command-dont-erase-buffer): New test.