[FIR] Save IR dumps to .ir.txt files instead of .txt in tests

This is needed to avoid clashes between different dumps from different
  handlers

compiler/testData/ir/irText/firProblems/SignatureClash.kt.txt

@@ -21,7 +21,7 @@ interface Base {
 }
 
 interface Delegate : Base {
-  abstract fun bar() 
+  abstract fun bar()
 
 }
 
@@ -75,4 +75,3 @@ data class DataClass : Derived, Delegate {
   }
 
 }
-