[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.descriptors.java

@@ -0,0 +1,11 @@
+public final class A /* A*/ {
+  private int z;
+
+  public  A();//  .ctor()
+
+  public final int getX();//  getX()
+
+  public final void setF(@org.jetbrains.annotations.NotNull() A);//  setF(A)
+
+  public final void setX(int);//  setX(int)
+}