checker: add immutable_builtin_modify.vv test (#9702)

vlib/v/checker/checker.v

@@ -1245,9 +1245,7 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) (string, token.Position) {
 				}
 				.array, .string {
 					// This should only happen in `builtin`
-					// TODO Remove `crypto.rand` when possible (see vlib/crypto/rand/rand.v,
-					// if `c_array_to_bytes_tmp` doesn't exist, then it's safe to remove it)
-					if c.file.mod.name !in ['builtin', 'crypto.rand'] {
+					if c.file.mod.name != 'builtin' {
 						c.error('`$typ_sym.kind` can not be modified', expr.pos)
 					}
 				}

vlib/v/checker/tests/immutable_builtin_modify.out

@@ -0,0 +1,11 @@
+vlib/v/checker/tests/immutable_builtin_modify.vv:2:3: error: `string` can not be modified
+    1 | s := ''
+    2 | s.len = 123
+      |   ~~~
+    3 | //
+    4 | b := []byte{}
+vlib/v/checker/tests/immutable_builtin_modify.vv:5:3: error: `array` can not be modified
+    3 | //
+    4 | b := []byte{}
+    5 | b.len = 34
+      |   ~~~

vlib/v/checker/tests/immutable_builtin_modify.vv

@@ -0,0 +1,5 @@
+s := ''
+s.len = 123
+//
+b := []byte{}
+b.len = 34