Move checkType directives for tests to separate folder

compiler/testData/diagnostics/helpers/types/checkType.kt

@@ -0,0 +1,7 @@
+package tests._checkType
+
+fun <T> checkSubtype(t: T) = t
+class Inv<T>
+fun <E> Inv<E>._() {}
+
+infix fun <T> T.checkType(f: Inv<T>.() -> Unit) {}