[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/classes/clashingFakeOverrideSignatures.fir.kt.txt

@@ -45,9 +45,9 @@ fun test(b: Base<String>, d: Derived, d2: Derived2) {
   b.foo(x = "")
   b.foo(y = "")
   d.foo(x = "")
-  d.foo(y = "")
+  d.foo(x = "")
+  d2.foo(x = "")
   d2.foo(x = "")
-  d2.foo(y = "")
 }
 
 open class BaseXY<X : Any?, Y : Any?> {