This commit adds a new `https_serve_directory()` test helper
and some custom self-signed and worthless certs that are used
during testing. They are not dynamically generated to avoid the
extra compuation time during tests (but they could be).
Generated via:
```
$ openssl req -new -newkey rsa:2048 -nodes -x509 \
-subj "/C=DE/ST=Berlin/L=Berlin/O=Org/CN=localhost" \
-keyout "key1.pem" -out "cert1.pem"
```
This will allow us to test `https` download URLs as well in e.g.
the curl source.
