test: update the test cert to expire in 100y

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.

test/data/certs/cert2.pem

@@ -1,21 +1,21 @@
 -----BEGIN CERTIFICATE-----
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 -----END CERTIFICATE-----