flag: add .usage_example/1, .footer/1, .remaining_parameters/0 and tests

vlib/flag/usage_example_test.v

@@ -0,0 +1,35 @@
+import os
+
+const the_source = 'vlib/flag/testdata/usage_example.v'
+
+const the_executable = os.real_path(os.join_path(os.cache_dir(), 'flag_usage_example_app.exe'))
+
+fn testsuite_begin() {
+	os.chdir(@VMODROOT)
+	os.rm(the_executable) or {}
+	res := os.execute('${@VEXE} -o $the_executable $the_source')
+	assert res.exit_code == 0
+	assert os.execute(the_executable).exit_code == 0
+	C.atexit(fn () {
+		os.rm(the_executable) or {}
+	})
+}
+
+fn normalise_lines(lines []string) string {
+	return '\n' + lines.join('\n')
+}
+
+fn check_program(opts string, extension string) {
+	result := the_source.replace('.v', extension)
+	res := os.execute('$the_executable $opts')
+	assert res.exit_code == 0
+	assert normalise_lines(res.output.split_into_lines()) == normalise_lines(os.read_lines(result) or {
+		panic(err)
+	})
+}
+
+fn test_normal_usage() {
+	check_program('abc def', '.out')
+	check_program(' --help', '.help.out')
+	check_program(' --version', '.version.out')
+}