Add another append test for basic_string

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@283331 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.modifiers/string_append/T_size_size.pass.cpp

@@ -164,6 +164,10 @@ int main()
     sv = s;
     s.append(sv, 0, std::string::npos);
     assert(s == "ABCDABCDABCDABCD");
+
+    sv = s;
+    s.append(sv, sv.size());
+    assert(s == "ABCDABCDABCDABCD");
     }
     
     {