[SLC] AbstractSymbolLightClassesByPsiTest: add more stable order

Now more diff between sources and libraries are readable.
Also prefer .descriptors to .fir as FIR is a first-class support

^KT-62892

compiler/testData/asJava/lightClasses/lightClassByPsi/jvmSynthetic.java

@@ -1,5 +1,7 @@
 public final class A /* A*/ {
-  private int z;
+  private int z = 1 /* initializer type: int */;
+
+  public A f;
 
   public  A();//  .ctor()