Test directory renamed to 'diagnostics' which reflects its contents

compiler/testData/diagnostics/checkerTestUtil/test.jet

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