Kotlin: add potentially static inner class test

Author: tamasvajk

java/ql/test/kotlin/query-tests/InnerClassCouldBeStatic/InnerClassCouldBeStatic.expected

@@ -0,0 +1 @@
+| Test.kt:3:5:5:5 | Companion | Companion should be made static, since the enclosing instance is not used. |

java/ql/test/kotlin/query-tests/InnerClassCouldBeStatic/InnerClassCouldBeStatic.qlref

@@ -0,0 +1 @@
+Performance/InnerClassCouldBeStatic.ql

java/ql/test/kotlin/query-tests/InnerClassCouldBeStatic/Test.kt

@@ -0,0 +1,6 @@
+class A {
+    fun fn1() {}
+    companion object {
+        fun fn2() {}
+    }
+}