add not found cases in the tests

vlib/builtin/fixed_array_index_test.v

@@ -1,35 +1,51 @@
 fn test_index_of_ints() {
 	ia := [1, 2, 3]!
-	ii := ia.index(2)
+	mut ii := ia.index(2)
 	dump(ii)
 	assert ii == 1
+
+	ii = ia.index(5)
+	dump(ii)
+	assert ii == -1
 }
 
 fn test_index_of_strings() {
 	sa := ['a', 'b', 'c']!
-	si := sa.index('b')
+	mut si := sa.index('b')
 	dump(si)
 	assert si == 1
+
+	si = sa.index('v')
+	dump(si)
+	assert si == -1
 }
 
 fn test_index_of_voidptrs() {
 	pa := [voidptr(123), voidptr(45), voidptr(99)]!
-	pi := pa.index(voidptr(45))
+	mut pi := pa.index(voidptr(45))
 	dump(pi)
 	assert pi == 1
-}
 
-//////////
+	pi = pa.index(unsafe { nil })
+	dump(pi)
+	assert pi == -1
+}
 
 fn a() {}
 
 fn b() {}
 
 fn c() {}
 
+fn v() {}
+
 fn test_index_of_fns() {
 	fa := [a, b, c]!
-	fi := fa.index(b)
+	mut fi := fa.index(b)
 	dump(fi)
 	assert fi == 1
+
+	fi = fa.index(v)
+	dump(fi)
+	assert fi == -1
 }