Move all basic completion tests to common folder

idea/testData/completion/basic/common/CompletionInSetter.kt

@@ -0,0 +1,10 @@
+val a: Int = 1
+    get() {
+      return $a
+    }
+    set(v) {
+      $a = <caret>
+    }
+
+
+// EXIST: a