os,tools: add os.vtmp_dir()

Use it to consistently place all temporary files created by tests in a overridable folder specific to the user, that is easy to cleanup later.

NOTE: os.temp_dir() on macos returns `/tmp`, and using `/tmp/v` is a problem when multiple unix users are trying to access/create/write to it.

vlib/os/os.v

@@ -719,6 +719,23 @@ pub fn temp_dir() string {
 	return path
 }
 
+// vtmp_dir returns the path to a folder, that is writable to V programs, *and* specific
+// to the OS user. It can be overriden by setting the env variable `VTMP`.
+pub fn vtmp_dir() string {
+	mut vtmp := getenv('VTMP')
+	if vtmp.len > 0 {
+		return vtmp
+	}
+	uid := getuid()
+	vtmp = join_path_single(temp_dir(), 'v_$uid')
+	if !exists(vtmp) || !is_dir(vtmp) {
+		// create a new directory, that is private to the user:
+		mkdir_all(vtmp, mode: 0o700) or { panic(err) }
+	}
+	setenv('VTMP', vtmp, true)
+	return vtmp
+}
+
 fn default_vmodules_path() string {
 	hdir := home_dir()
 	res := join_path_single(hdir, '.vmodules')