[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/jvmSyntheticForAccessors.java

@@ -1,4 +1,8 @@
 public final class X /* X*/ {
+  private int v = 1 /* initializer type: int */;
+
+  private int w = 1 /* initializer type: int */;
+
   public  X();//  .ctor()
 
   public final int getW();//  getW()