test(1): quote closing bracket in error message

If test is called as [ and one forgets to close the bracket, the error
message is currently
[: missing ]

To make it obvious that this is not something printed in brackets,
quote the closing bracket in the message, which is what everybody else
is doing:
[: missing ']'

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1346

bin/test/test.c

@@ -198,7 +198,7 @@ main(int argc, char **argv)
 		p++;
 	if (strcmp(p, "[") == 0) {
 		if (strcmp(argv[--argc], "]") != 0)
-			error("missing ]");
+			error("missing ']'");
 		argv[argc] = NULL;
 	}