[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/smartCasts/varnotnull/varCheck.fir.kt

@@ -0,0 +1,9 @@
+fun get(): String? {
+    return ""
+}
+
+fun foo(): Int {
+    var c: String? = get()
+    c!!.length
+    return c.length // Previous line should make !! unnecessary here.
+}