Fix another test/[ corner case bug; add --posix test script

This fixes another corner case bug in the horror show that is the
test/[ comand.

Reproducer:

   $ ksh --posix -c 'test X -a -n'
   ksh: test: argument expected

Every other shell returns 0 (success) as, POSIXly, this is a test
for the strings 'X' and '-n' both being non-empty, combined with
the binary -a (logical and) operator. Instead, '-n' was taken as a
unary primary operator with a missing argument, which is incorrect.

POSIX reference:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html
> 3 arguments:
> * If $2 is a binary primary, perform the binary test of $1 and $3.

src/cmd/ksh93/bltins/test.c:
- e3(): If the final argument begins with a dash, always treat it
  as a test for a non-empty string, therefore return true. Do not
  limit this to "new flags" only.

src/cmd/ksh93/tests/posix.sh:
- Added. These are tests for every aspect of the POSIX mode.

NEWS

@@ -3,6 +3,10 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-03-10:
+
+- Fixed another corner case bug in the 'test'/'[' command.
+
 2022-03-05:
 
 - The 'enum' command can now create more than one type per invocation.