Got rid of "jet" file extension.

compiler/testData/diagnostics/checkerTestUtil/test.kt

@@ -0,0 +1,15 @@
+fun foo(u : Unit) : Int = 1
+
+fun test() : Int {
+    foo(1)
+    val a : () -> Unit = {
+        foo(1)
+    }
+    return 1 - "1"
+}
+
+class A() {
+    val x : Int = foo1(xx)
+}
+
+fun foo1() {}