test: accept "==" as a synonym for "="

test: accept "==" as a synonym for "="

Make GNU coreutils' test recognize "==" as a synonym for "=".
This is already the case in GNU coreutils' expr, bash, ksh,
busybox ash, FreeBSD-current /bin/sh and /bin/test, and
OpenBSD's /bin/sh.

Before, env test a '==' a would fail with this diagnostic:
"test: ==: binary operator expected".  Now, it succeeds.
* src/test.c: Accept "==" as a synonym for "=".
* doc/coreutils.texi (String tests): Document it.
Reported as http://debbugs.gnu.org/8263
Also see http://austingroupbugs.net/view.php?id=375