Do not mark kotlin.test.Test annotated declarations as unused

So #KT-20523 Fixed

idea/testData/multiFileLocalInspections/unusedSymbol/fromKotlinTestClass/after/kotlin/test/KotlinTest.kt

@@ -0,0 +1,6 @@
+package kotlin.test
+
+annotation class Test
+
+fun assertEquals(expected: Any, actual: Any) {}
+

idea/testData/multiFileLocalInspections/unusedSymbol/fromKotlinTestClass/after/test.kt

@@ -0,0 +1,10 @@
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class SomeTest {
+
+    @Test
+    fun testSomething() {
+        assertEquals(4, 2 + 2)
+    }
+}