[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/expressions/whenElse.ir.txt

@@ -0,0 +1,8 @@
+FILE fqName:<root> fileName:/whenElse.kt
+  FUN name:test visibility:public modality:FINAL <> () returnType:kotlin.Int
+    BLOCK_BODY
+      RETURN type=kotlin.Nothing from='public final fun test (): kotlin.Int declared in <root>'
+        WHEN type=kotlin.Int origin=WHEN
+          BRANCH
+            if: CONST Boolean type=kotlin.Boolean value=true
+            then: CONST Int type=kotlin.Int value=42