all: Grand test refactor (fixes #8779, fixes #8799)

This fixes various test issues with Go 1.20.

- Most tests rewritten to use fakefs where possible
- Some tests that were already skipped, or dubious (invasive,
  unmaintainable, unclear what they even tested) have been removed
- Some actual code rewritten to better support testing in fakefs

Co-authored-by: Eric P <eric@kastelo.net>

lib/model/testos_test.go

@@ -7,9 +7,6 @@
 package model
 
 import (
-	"os"
-	"time"
-
 	"github.com/syncthing/syncthing/lib/fs"
 )
 
@@ -19,10 +16,6 @@ type fatal interface {
 	Helper()
 }
 
-type fatalOs struct {
-	fatal
-}
-
 func must(f fatal, err error) {
 	f.Helper()
 	if err != nil {
@@ -36,56 +29,3 @@ func mustRemove(f fatal, err error) {
 		f.Fatal(err)
 	}
 }
-
-func (f *fatalOs) Chmod(name string, mode os.FileMode) {
-	f.Helper()
-	must(f, os.Chmod(name, mode))
-}
-
-func (f *fatalOs) Chtimes(name string, atime time.Time, mtime time.Time) {
-	f.Helper()
-	must(f, os.Chtimes(name, atime, mtime))
-}
-
-func (f *fatalOs) Create(name string) *os.File {
-	f.Helper()
-	file, err := os.Create(name)
-	must(f, err)
-	return file
-}
-
-func (f *fatalOs) Mkdir(name string, perm os.FileMode) {
-	f.Helper()
-	must(f, os.Mkdir(name, perm))
-}
-
-func (f *fatalOs) MkdirAll(name string, perm os.FileMode) {
-	f.Helper()
-	must(f, os.MkdirAll(name, perm))
-}
-
-func (f *fatalOs) Remove(name string) {
-	f.Helper()
-	if err := os.Remove(name); err != nil && !os.IsNotExist(err) {
-		f.Fatal(err)
-	}
-}
-
-func (f *fatalOs) RemoveAll(name string) {
-	f.Helper()
-	if err := os.RemoveAll(name); err != nil && !os.IsNotExist(err) {
-		f.Fatal(err)
-	}
-}
-
-func (f *fatalOs) Rename(oldname, newname string) {
-	f.Helper()
-	must(f, os.Rename(oldname, newname))
-}
-
-func (f *fatalOs) Stat(name string) os.FileInfo {
-	f.Helper()
-	info, err := os.Stat(name)
-	must(f, err)
-	return info
-}