checker: add a test for the single letter receiver

cmd/tools/vtest-self.v

@@ -95,7 +95,7 @@ fn main() {
 	args := os.args.clone()
 	args_string := args[1..].join(' ')
 	cmd_prefix := args_string.all_before('test-self')
-	title := 'testing all fixed tests'
+	title := 'testing all tests'
 	all_test_files := os.walk_ext(os.join_path(vroot, 'vlib'), '_test.v')
 	testing.eheader(title)
 	mut tsession := testing.new_test_session(cmd_prefix)

vlib/v/checker/tests/receiver_unknown_type_single_letter.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/receiver_unknown_type_single_letter.vv:1:5: error: unknown type `A`
+    1 | fn (p A) foo() {}
+      |     ~~~

vlib/v/checker/tests/receiver_unknown_type_single_letter.vv

@@ -0,0 +1,2 @@
+fn (p A) foo() {}
+