golang_str: tests: Fix test_strings_methods wrt upcoming str=bstr and unicode=ustr

When gpython will start patching builtin str and unicode types with bstr
and ustr the first argument to assertDeepEQ will have builtin str or
unicode type and the existing

    assert not isinstance(a, (bstr, ustr))

will break.

-> Rewrite that assert to do equivalent check carefully that does not
break when str/unicode types are patched with bstr and ustr.

golang/golang_str_test.py

@@ -1713,7 +1713,11 @@ def test_strings_methods():
         ur = xcall(us, meth, *argv, **kw)
 
         def assertDeepEQ(a, b, bstrtype):
-            assert not isinstance(a, (bstr, ustr))
+            # `assert not isinstance(a, (bstr, ustr))` done carefully not to
+            # break when bytes/unicode are patched with bstr/ustr
+            if isinstance(a, bytes):    assert type(a) is bytes
+            if isinstance(a, unicode):  assert type(a) is unicode
+
             if type(a) is unicode:
                 assert type(b) is bstrtype
                 assert a == b