Fixes #228 Unit tests should stop calling sudo kill

I have removed this code completely. If it turns out to be a problem
then we can investigate a solution that does not require executing shell
commands as root.
Fixes #229 Skip hsps_no_hsps_header_over_https if stunnel4 is not installed

Also simplifies stunnel process management. Properly terminate stunnel.
Fixes #230 Fix spelling mistakes in test_hsts