FIR: dump nested classes in loadJava tests

Because those test are single Java files, many of them use a top-level
class as a container for multiple classes to be tested. Such tests do
almost nothing if those nested classes aren't handled.

compiler/testData/loadJava/compiledJava/kotlinSignature/StarProjection.fir.txt

@@ -3,4 +3,6 @@ public final class StarProjection : R|kotlin/Any| {
 
     public constructor(): R|test/StarProjection|
 
+    public abstract interface MyClass<T : R|ft<kotlin/CharSequence, kotlin/CharSequence?>|> : R|kotlin/Any| {
+    }
 }