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