[BUILTIN] Correctly handle test ! ! = !

This patch adds a special case in testcmd for the 4-argument
expression beginning with a !.  Without this ! ! = ! is deemed
a syntax error, which breaks POSIX.

Note that this special case does not extend down into subexpressions
so if ! ! = ! is used inside parentheses then a syntax error will
still occur as before.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>