tests: add tests for checking fixed array .sort() errors

vlib/v/checker/tests/fixed_array_sort_err.out

@@ -0,0 +1,45 @@
+vlib/v/checker/tests/fixed_array_sort_err.vv:3:6: error: expected 0 or 1 argument, but got 2
+    1 | fn main() {
+    2 |     mut arr := [3, 2, 1]!
+    3 |     arr.sort(a < b, a)
+      |         ~~~~~~~~~~~~~~
+    4 |     arr.sort(a == b)
+    5 |     arr.sort(a > a)
+vlib/v/checker/tests/fixed_array_sort_err.vv:4:6: error: `.sort()` can only use `<` or `>` comparison
+    2 |     mut arr := [3, 2, 1]!
+    3 |     arr.sort(a < b, a)
+    4 |     arr.sort(a == b)
+      |         ~~~~~~~~~~~~
+    5 |     arr.sort(a > a)
+    6 |     arr.sort(c > d)
+vlib/v/checker/tests/fixed_array_sort_err.vv:5:6: error: `.sort()` cannot use same argument
+    3 |     arr.sort(a < b, a)
+    4 |     arr.sort(a == b)
+    5 |     arr.sort(a > a)
+      |         ~~~~~~~~~~~
+    6 |     arr.sort(c > d)
+    7 | }
+vlib/v/checker/tests/fixed_array_sort_err.vv:6:11: error: can not access external variable `c`
+    4 |     arr.sort(a == b)
+    5 |     arr.sort(a > a)
+    6 |     arr.sort(c > d)
+      |              ^
+    7 | }
+vlib/v/checker/tests/fixed_array_sort_err.vv:6:6: error: `.sort()` can only use `a` or `b` as argument, e.g. `arr.sort(a < b)`
+    4 |     arr.sort(a == b)
+    5 |     arr.sort(a > a)
+    6 |     arr.sort(c > d)
+      |         ~~~~~~~~~~~
+    7 | }
+vlib/v/checker/tests/fixed_array_sort_err.vv:6:11: error: undefined ident: `c`
+    4 |     arr.sort(a == b)
+    5 |     arr.sort(a > a)
+    6 |     arr.sort(c > d)
+      |              ^
+    7 | }
+vlib/v/checker/tests/fixed_array_sort_err.vv:6:15: error: undefined ident: `d`
+    4 |     arr.sort(a == b)
+    5 |     arr.sort(a > a)
+    6 |     arr.sort(c > d)
+      |                  ^
+    7 | }

vlib/v/checker/tests/fixed_array_sort_err.vv

@@ -0,0 +1,7 @@
+fn main() {
+	mut arr := [3, 2, 1]!
+	arr.sort(a < b, a)
+	arr.sort(a == b)
+	arr.sort(a > a)
+	arr.sort(c > d)
+}

vlib/v/checker/tests/fn_return_fixed_array_sort_err.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/fn_return_fixed_array_sort_err.vv:6:14: error: the `sort()` method can be called only on mutable receivers, but `ret_array()` is a call expression
+    4 | 
+    5 | fn main() {
+    6 |     ret_array().sort()
+      |                 ~~~~~~
+    7 | }
+vlib/v/checker/tests/fn_return_fixed_array_sort_err.vv:6:2: error: cannot pass expression as `mut`
+    4 | 
+    5 | fn main() {
+    6 |     ret_array().sort()
+      |     ~~~~~~~~~~~
+    7 | }

vlib/v/checker/tests/fn_return_fixed_array_sort_err.vv

@@ -0,0 +1,7 @@
+fn ret_array() [3]int {
+	return [1, 3, 2]!
+}
+
+fn main() {
+	ret_array().sort()
+}