dl: add a complete tested shared library generation/usage example

vlib/dl/example/use_test.v

@@ -0,0 +1,52 @@
+module main
+
+import os
+import dl
+
+const (
+	vexe              = os.real_path(os.getenv('VEXE'))
+	cfolder           = os.dir(@FILE)
+	so_ext            = dl.dl_ext
+	library_file_name = os.join_path(cfolder, 'library$so_ext')
+)
+
+fn test_vexe() {
+	eprintln('vexe: $vexe')
+	assert vexe != ''
+	eprintln('os.executable: ' + os.executable())
+	eprintln('@FILE: ' + @FILE)
+	eprintln('cfolder: $cfolder')
+	eprintln('so_ext: $so_ext')
+	eprintln('library_file_name: $library_file_name')
+}
+
+fn test_can_compile_library() {
+	os.chdir(cfolder)
+	os.rm(library_file_name)
+	res := v_compile('-d no_backtrace -o library -shared library.v')
+	eprintln('res: $res')
+	assert os.is_file(library_file_name)
+}
+
+fn test_can_compile_main_program() {
+	os.chdir(cfolder)
+	assert os.is_file(library_file_name)
+	result := v_compile('run use.v')
+	eprintln('result: $result')
+	assert result.output.contains('res: 4')
+	os.rm(library_file_name)
+}
+
+fn v_compile(vopts string) os.Result {
+	cmd := '"$vexe" -showcc $vopts'
+	eprintln('>>> v_compile cmd: $cmd')
+	res := os.exec(cmd) or { panic(err) }
+	eprintln('>>> v_compile res: $res')
+	// assert res.exit_code == 0
+	$if !windows {
+		os.system('dir $cfolder -a -l')
+	} $else {
+		os.system('dir $cfolder /a')
+	}
+	return res
+}