Revert to running tests from from a temp dir when test-sources is unset (#2420)

* Revert to running tests from from a temp dir when test-sources is unset

* Fix placeholders error message, add test for it

* Add back {project} placeholders to CIBW_TEST_COMMAND in tests & docs

* Update test/test_before_test.py

---------

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

test/test_macos_archs.py

@@ -42,11 +42,9 @@ def test_cross_compiled_build(tmp_path):
 @pytest.mark.parametrize(
     "test_config",
     [
-        # Run the test suite in the project folder
         {
             "CIBW_TEST_COMMAND": '''python -c "import platform; print('running tests on ' + platform.machine())"''',
         },
-        # Nominate the set of test sources to copy
         {
             "CIBW_TEST_COMMAND": "python tests/test_suite.py",
             "CIBW_TEST_SOURCES": "tests",