tests: add cmd/tools/vdoc/tests/vdoc_file_test.v, extract a v.util.diff module, cleanup

cmd/tools/vdoc/tests/testdata/project1/main.out

@@ -0,0 +1,3 @@
+cmd/tools/vdoc/tests/testdata/project1/main.v:0:1: error: unexpected unknown, expecting `const`
+    1 | const (
+    2 |     source_root = 'temp'

cmd/tools/vdoc/tests/testdata/project1/main.v

@@ -0,0 +1,8 @@
+const (
+	source_root = 'temp'
+)
+
+// funky - comment for function below
+fn funky() {
+	println('hi')
+}

cmd/tools/vdoc/tests/vdoc_file_test.v

@@ -0,0 +1,72 @@
+import os
+import rand
+import term
+import v.util.vtest
+import v.util.diff
+
+const vexe = @VEXE
+
+const vroot = @VMODROOT
+
+const diff_cmd = find_diff_cmd()
+
+fn find_diff_cmd() string {
+	return diff.find_working_diff_command() or { '' }
+}
+
+fn test_vet() {
+	os.setenv('VCOLORS', 'never', true)
+	os.chdir(vroot)
+	test_dir := 'cmd/tools/vdoc/tests/testdata'
+	main_files := get_main_files_in_dir(test_dir)
+	fails := check_path(vexe, test_dir, main_files)
+	assert fails == 0
+}
+
+fn get_main_files_in_dir(dir string) []string {
+	mut mfiles := os.walk_ext(dir, '.v')
+	mfiles.sort()
+	return mfiles
+}
+
+fn check_path(vexe string, dir string, tests []string) int {
+	mut nb_fail := 0
+	paths := vtest.filter_vtest_only(tests, basepath: vroot)
+	for path in paths {
+		program := path
+		print(path + ' ')
+		res := os.execute('$vexe doc $program')
+		if res.exit_code < 0 {
+			panic(res.output)
+		}
+		mut expected := os.read_file(program.replace('main.v', 'main.out')) or { panic(err) }
+		expected = clean_line_endings(expected)
+		found := clean_line_endings(res.output)
+		if expected != found {
+			println(term.red('FAIL'))
+			println('============')
+			println('expected:')
+			println(expected)
+			println('============')
+			println('found:')
+			println(found)
+			println('============\n')
+			println('diff:')
+			println(diff.color_compare_strings(diff_cmd, rand.ulid(), found, expected))
+			println('============\n')
+			nb_fail++
+		} else {
+			println(term.green('OK'))
+		}
+	}
+	return nb_fail
+}
+
+fn clean_line_endings(s string) string {
+	mut res := s.trim_space()
+	res = res.replace(' \n', '\n')
+	res = res.replace(' \r\n', '\n')
+	res = res.replace('\r\n', '\n')
+	res = res.trim('\n')
+	return res
+}