Add deserialization tests

[jiholee17]

Add basic tests for deserialization of JSON response → Java POJO, factored out into a separate JavaDeserializationTest.kt file for extensibility.

Also refactor CodegenTestClassLoader that loads and links test classes to use HashMap and synchronized block instead of ConcurrentHashMap as the latter was causing

Recursive update
java.lang.IllegalStateException: Recursive update
	at java.base/java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1763)
	at com.netflix.graphql.dgs.codegen.CodegenTestClassLoader.loadClass(CodegenTestClassLoader.kt:38)
	at java.base/java.lang.ClassLoader.defineClass1(Native Method)
	at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1017)
	at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:879)
	at com.netflix.graphql.dgs.codegen.CodegenTestClassLoader.loadClass$lambda$4$lambda$2(CodegenTestClassLoader.kt:51)
	at com.netflix.graphql.dgs.codegen.CodegenTestClassLoader.loadClass$lambda$4$lambda$3(CodegenTestClassLoader.kt:44)
	at java.base/java.util.Optional.map(Optional.java:260)
	at com.netflix.graphql.dgs.codegen.CodegenTestClassLoader.loadClass$lambda$4(CodegenTestClassLoader.kt:44)
	at com.netflix.graphql.dgs.codegen.CodegenTestClassLoader.loadClass$lambda$5(CodegenTestClassLoader.kt:38)
	at java.base/java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1708)
	at com.netflix.graphql.dgs.codegen.CodegenTestClassLoader.loadClass(CodegenTestClassLoader.kt:38)

when loadClass is called for a non-bootstrap superclass while evaluating child class (i.e. *Projection and BaseSubProjectionNode) when it re-enters the map, to set up instantiating instance for runtime test.