fmt: add keep tests for struct init, fix warnings

vlib/v/fmt/tests/import_single_expected.vv

@@ -1,4 +1,5 @@
 import os
 
 fn main() {
+	os.system('echo hi')
 }

vlib/v/fmt/tests/import_single_input.vv

@@ -1,4 +1,5 @@
 import os
 
 fn main() {
+	os.system('echo hi')
 }

vlib/v/fmt/tests/struct_init_keep.vv

@@ -0,0 +1,11 @@
+struct User {
+	age  int
+	name string
+}
+
+fn main() {
+	u := User{
+		age: 54
+	}
+	println(u)
+}