fix: make KsqlAvroSerializerTest work with Java 16

[vvcephei]

Apparently some small things around casting behavior
and error messages have changed. This change makes the
test more generic so that it works regardless of the JVM.

Testing done

Ran the tests on my machine. If they also pass in Jenkins, it should be fine to merge.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[ghost]

@confluentinc It looks like @vvcephei just signed our Contributor License Agreement. 👍

Always at your service,

clabot

[agavra]

LGTM, but for my own education - why are we running this with Jdk16? I'm pretty sure lots of things break at that language level

[vvcephei]

Thanks, @agavra . It's not a matter of the compilation language level. I just happen to have the Java 16 runtime on my computer. The exception is fine, but the string that it returns in the exception varies across JVMs. Since the exception message is not in the JVM spec, it seems like we shouldn't write tests that rely on exact phrasing.