[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/singletons/companion.kt.txt

@@ -22,4 +22,3 @@ class Z {
   }
 
 }
-

compiler/testData/ir/irText/singletons/enumEntry.fir.kt.txt

@@ -36,4 +36,3 @@ open enum class Z : Enum<Z> {
   fun valueOf(value: String): Z /* Synthetic body for ENUM_VALUEOF */
 
 }
-

compiler/testData/ir/irText/singletons/enumEntry.kt.txt

@@ -36,4 +36,3 @@ open enum class Z : Enum<Z> {
   fun valueOf(value: String): Z /* Synthetic body for ENUM_VALUEOF */
 
 }
-

compiler/testData/ir/irText/singletons/object.kt.txt

@@ -22,4 +22,3 @@ object Z {
   }
 
 }
-