Add more complete Asn1Strings validation tests

String validation tests in [asn1string.json](https://github.com/a-sit-plus/signum/pull/345/files/1e5787fb817fd45289ae506eb80b4515bfb9c5bf#diff-186a5991119eeb45d457393a2d61fd4f981ac0e7ce8098769a87297878bb1bdc) are currently very shallow. We should add more systematic and complete string validation tests.
See https://github.com/a-sit-plus/signum/pull/345#discussion_r2330349501