Motivation:
Some JDKs dissallow the usage of keysizes < 2048, so we should not use such small keysizes in tests.
This showed up on fedora 32:
```
Caused by: java.security.cert.CertPathValidatorException: Algorithm constraints check failed on keysize limits. RSA 1024bit key used with certificate: CN=tlsclient. Usage was tls client
at sun.security.util.DisabledAlgorithmConstraints$KeySizeConstraint.permits(DisabledAlgorithmConstraints.java:817)
at sun.security.util.DisabledAlgorithmConstraints$Constraints.permits(DisabledAlgorithmConstraints.java:419)
at sun.security.util.DisabledAlgorithmConstraints.permits(DisabledAlgorithmConstraints.java:167)
at sun.security.provider.certpath.AlgorithmChecker.check(AlgorithmChecker.java:326)
at sun.security.provider.certpath.PKIXMasterCertPathValidator.validate(PKIXMasterCertPathValidator.java:125)
... 23 more
```
Modifications:
Replace hardcoded keys / certs with SelfSignedCertificate
Result:
No test-failures related to small key sizes anymore.