bpo-37473: Don't import importlib ASAP in tests (GH-14661)

bpo-15386, bpo-37473: test_import, regrtest and libregrtest no longer
import importlib as soon as possible, as the first import, "to test
bpo-15386".

It is tested by test_import.test_there_can_be_only_one().

Sort test_import imports.