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_pyodide.py

@@ -129,7 +129,7 @@ def test_pyodide_build_and_test(tmp_path, expect_failure):
             add_args=["--platform", "pyodide"],
             add_env={
                 "CIBW_TEST_REQUIRES": "pytest",
-                "CIBW_TEST_COMMAND": "python -m pytest",
+                "CIBW_TEST_COMMAND": "python -m pytest {project}",
             },
         )
         # check that the expected wheels are produced