* runtest.exp: Handle multiple directories in TCL variables

* runtest.exp: Handle multiple directories in TCL variables
dir_to_run and cmdline_dir_to_run; limit *.exp find to
one directory level to avoid foo/bar/baz.exp getting tested
twice (when ${dir} is 'foo', and when ${dir} is 'foo/bar').

* lib/utils.exp (find): Add support for a -maxdepth <n>
option to limit find to <n> directories deep.