KT-6810 Don't report warnings in dummy.kt

#KT-6810 Fixed

compiler/testData/cli/jvm/warningsInDummy.args

@@ -0,0 +1,3 @@
+$TESTDATA_DIR$/warningsInDummy.kt
+-d
+$TEMP_DIR$

compiler/testData/cli/jvm/warningsInDummy.kt

@@ -0,0 +1,5 @@
+fun foo(s: String) {
+    if (s == "") {
+
+    }
+}

compiler/testData/cli/jvm/warningsInDummy.out

@@ -0,0 +1 @@
+OK