Check for compilation warnings in `SimpleSerialBuilderCodeGenTest`

[Marcono1234]

See OpenHFT/Java-Runtime-Compiler#93

Additionally, because CompilerUtils.CACHED_COMPILER compiles with -g -nowarn by default, consider using CachedCompiler(File, File, List<String>) either with an empty list of javac options, or with -Xlint:all (unless that causes unrelated warnings which are not useful for the test)?

[Marcono1234]

Could consider switching to https://github.com/ascopes/java-compiler-testing