test foo =~ foo should fail with exit status 2 (#245)

When test is passed the '=~' operator, it will silently fail with
exit status 1:
    $ test foo =~ foo; echo $?
    1
This bug is caused by test_binop reaching the 'NOTREACHED' area of
code. The bugfix was adapted from ksh2020:
https://github.com/att/ast/issues/1152

src/cmd/ksh93/bltins/test.c: test_binop():
- Error out with a message suggesting usage of '[[ ... ]]' if '=~'
  is passed to the test builtin.
- Special-case TEST_END (']]') as that is not really an operator.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

NEWS

@@ -3,6 +3,13 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-03-27:
+
+- The 'test' builtin will now show an error message when given the invalid ']]'
+  or '=~' operators; it also properly returns with exit status 2 now (instead
+  of exit status 1). If the invalid operator is supported by [[ ... ]] (such
+  as '=~'), test will now suggest the usage of [[ ... ]] instead.
+
 2021-03-22:
 
 - A new --globcasedetect shell option is added to ksh on OSs where we can check