analysis/facts/nilness: limit slice to array pointer tests to Go 1.17+

(cherry picked from commit 51b6efb)

Author: dominikh

analysis/facts/nilness/testdata/src/Nilness/Nilness.go

@@ -101,39 +101,3 @@ func fn20() *int {
 	}
 	return nil
 }
-
-func fn21() *[5]int { // want fn21:`never returns nil: \[never\]`
-	var x []int
-	return (*[5]int)(x)
-}
-
-func fn22() *[0]int {
-	var x []int
-	return (*[0]int)(x)
-}
-
-func fn23() *[5]int { // want fn23:`never returns nil: \[never\]`
-	var x []int
-	type T [5]int
-	ret := (*T)(x)
-	return (*[5]int)(ret)
-}
-
-func fn24() *[0]int {
-	var x []int
-	type T [0]int
-	ret := (*T)(x)
-	return (*[0]int)(ret)
-}
-
-func fn25() *[5]int { // want fn25:`never returns nil: \[never\]`
-	var x []int
-	type T *[5]int
-	return (T)(x)
-}
-
-func fn26() *[0]int {
-	var x []int
-	type T *[0]int
-	return (T)(x)
-}