tests: use os.vtmp_dir() in autofree_toml.vv, so it can be cleaned up automatically; simplify the code

vlib/v/gen/c/testdata/autofree_toml.vv

@@ -2,14 +2,8 @@
 import toml
 import os
 
-fn main() {
+config_fname := os.join_path(os.vtmp_dir(), 'config.toml')
-	config_fname := 'config.toml'
+tab_title := 'test tab title'
-	tab_title := 'test tab title'
+os.write_file(config_fname, 'tab_title = "${tab_title}"')!
-	if !os.exists(config_fname) {
+doc := toml.parse_file(config_fname)!
-		mut f := os.create(config_fname) or { panic(err) }
+assert doc.value('tab_title').string() == tab_title
-		f.writeln('tab_title = "${tab_title}"') or { panic(err) }
-		f.close()
-	}
-	doc := toml.parse_file(config_fname) or { panic(err) }
-	assert doc.value('tab_title').string() == tab_title
-}