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>

src/cmd/ksh93/include/test.h

@@ -66,6 +66,7 @@ extern const char	test_opchars[];
 extern const char	e_argument[];
 extern const char	e_missing[];
 extern const char	e_badop[];
+extern const char	e_unsupported_op[];
 extern const char	e_tstbegin[];
 extern const char	e_tstend[];