Update testvalueflow.cpp

Author: chrchr-github

test/testvalueflow.cpp

@@ -3112,6 +3112,12 @@ class TestValueFlow : public TestFixture {
                "    return x;\n"
                "}\n";
         ASSERT_EQUALS(false, testValueOfXKnown(code, 9U, 1));
+
+        code = "int32_t f() {\n"
+               "    const int32_t x = 0xffffffff;\n"
+               "    return x;\n"
+               "}\n";
+        ASSERT_EQUALS(true, testValueOfXKnown(code, 3U, -1));
     }
 
     void valueFlowAfterSwap()