When generating the original test certs no `-days` paramter was
passed which resulted in a too low `notAfter` value.
This commit fixes this and uses 100y also updates the README:
```
$ openssl x509 -enddate -noout -in test/data/certs/cert1.pem
notAfter=Aug 2 10:42:40 2124 GMT
$ openssl x509 -enddate -noout -in test/data/certs/cert2.pem
notAfter=Aug 2 10:42:45 2124 GMT
```
This fixes a test failure in https://github.com/osbuild/osbuild/pull/1819
for the `test_curl_download_many_mixed_certs` test.
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.
