pytest: fixes for recent python, add FTP tests

Fixes:
- in uds tests, abort also silently on os errors
- be conservative on the h3 goaway duration
- detect curl debug build and use in checks
- fix caddy version check for slight difference under linux
- set caddy default path fitting for linux
- fix deprecation warnings in valid time checks

FTP tests:
- add '--with-test-vsftpd=path' to configure
- use vsftpd default path suitable for linux
- add test_30 with plain FTP tests
- add test_31 with --ssl-reqd FTP tests
- add vsftpd to linux GHA for pytest workflows

Closes #13661

.github/workflows/linux.yml

@@ -383,7 +383,7 @@ jobs:
 
       - if: contains(matrix.build.install_steps, 'pytest')
         run: |
-          sudo apt-get install apache2 apache2-dev libnghttp2-dev
+          sudo apt-get install apache2 apache2-dev libnghttp2-dev vsftpd
           sudo python3 -m pip install -r tests/http/requirements.txt
         name: 'install pytest and apach2-dev'