tests: sort import lines in tinyproxy.py

tests: sort import lines in failfilemerge.py

tests: sort import lines in dumbhttp.py This series is an attempt to enable import-checker.py for tests/**.py, but it turned out not easy. Since many tests have been ported to absolute_import without the coverage, import-checker.py reports a lot of errors right now. Should we enable import-checker.py without fixing all of them so that we won't get more errors?