btrfs-progs: tests: make sure _is_file_or_command does not get confused

The test cli/007-check-force reports something like:

$ type -p '--string that starts with dashes'
bash: type: --: invalid option

Add the option/argument separator.

Signed-off-by: David Sterba <dsterba@suse.com>

tests/common

@@ -35,7 +35,7 @@ _is_file_or_command()
 	if [ -f "$msg" -o -d "$msg" -o -b "$msg" ]; then
 		return 0
 	fi
-	msg=$(type -p "$msg")
+	msg=$(type -p -- "$msg")
 	if [ -f "$msg" ]; then
 		return 0
 	fi