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/tests/builtins.sh

@@ -996,6 +996,32 @@ then	got=$( { "$SHELL" -c '
 		"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
 fi
 
+# ==========
+# Verify that the POSIX 'test' builtin complains loudly when the '=~' operator is used rather than
+# failing silently. See https://github.com/att/ast/issues/1152.
+actual=$($SHELL -c 'test foo =~ foo' 2>&1)
+actual_status=$?
+actual=${actual#*: }
+expect='test: =~: operator not supported; use [[...]]'
+expect_status=2
+[[ "$actual" = "$expect" ]] || err_exit "test =~ failed (expected $expect, got $actual)"
+[[ "$actual_status" = "$expect_status" ]] ||
+    err_exit "test =~ failed with the wrong exit status (expected $expect_status, got $actual_status)"
+
+# Invalid operators 'test' and '[[...]]' both reject should also cause an error with exit status 2.
+for operator in '===' ']]'
+do
+	actual="$($SHELL -c "test foo $operator foo" 2>&1)"
+	actual_status=$?
+	actual=${actual#*: }
+	expect="test: $operator: unknown operator"
+	expect_status=2
+	[[ "$actual" = "$expect" ]] || err_exit "test $operator failed" \
+		"(expected $(printf %q "$expect"), got $(printf %q "$actual"))"
+	[[ "$actual_status" = "$expect_status" ]] ||
+		err_exit "'test foo $operator foo' failed with the wrong exit status (expected $expect_status, got $actual_status)"
+done
+
 # ======
 # Regression test for https://github.com/att/ast/issues/1402
 #