[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/whenWithSubjectVariable.fir.kt.txt

@@ -1,18 +0,0 @@
-fun foo(): Any {
-  return 1
-}
-
-fun test(): Int {
-  return { // BLOCK
-    val y: Any = foo()
-    when {
-      EQEQ(arg0 = y, arg1 = 42) -> 1
-      y is String -> y /*as String */.<get-length>()
-      y !is Int -> 2
-      error("") /* ErrorCallExpression */y;  -> 3
-      error("") /* ErrorCallExpression */y; .not() -> 4
-      else -> -1
-    }
-  }
-}
-