Removing test_select_caster_alt completely.

For full background see: #3931

TL;DR: The work under PR #3931 made it obvious that test_select_caster_alt was too simple for deriving meaningful conclusions.

Author: rwgk

tests/CMakeLists.txt

@@ -149,7 +149,6 @@ set(PYBIND11_TEST_FILES
     test_pickling
     test_pytypes
     test_select_caster
-    test_select_caster_alt
     test_sequences_and_iterators
     test_smart_ptr
     test_stl

tests/test_select_caster.py

@@ -1,7 +1,6 @@
 import pytest
 
 from pybind11_tests import select_caster as m
-from pybind11_tests import select_caster_alt as m_alt
 
 
 def test_mock_casters():
@@ -10,10 +9,6 @@ def test_mock_casters():
     assert m.unnamed_ns_num() == 303
 
 
-def test_mock_casters_alt():
-    assert m_alt.have_a_ns_num() == 121
-
-
 def test_minimal_real_caster():
     assert m.mrc_return() == 1505
     assert m.mrc_arg("ignored") == 2404