Make a test farm tests package (#8821)

Fixes https://github.com/certbot/certbot/issues/8781.

This PR makes our test farm tests into a normal package so it and its dependencies can be tracked and installed like our other packages.

Other noteworthy changes in this PR:

* Rather than continuing to place logs in your CWD, they're placed in a temporary directory that is printed to the terminal.
*  `tests/letstest/auto_targets.yaml` was deleted rather than renamed because the file is no longer used.

* make a letstest package

* remove deleted deps

* fix letstest install

* add __init__.py

* call main

* Explicitly mention activating venv

* rerename file

* fix version.py path

* clarify "this"

* Use >= instead of caret requirement

tools/venv.py

@@ -44,6 +44,7 @@ REQUIREMENTS = [
     '-e certbot-nginx',
     '-e certbot-compatibility-test',
     '-e certbot-ci',
+    '-e letstest',
 ]
 
 if sys.platform == 'win32':