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/config/folderconfiguration.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"path"
+	"path/filepath"
 	"sort"
 	"strings"
 	"time"
@@ -90,11 +91,11 @@ func (f *FolderConfiguration) CreateMarker() error {
 		return nil
 	}
 
-	permBits := fs.FileMode(0777)
+	permBits := fs.FileMode(0o777)
 	if build.IsWindows {
 		// Windows has no umask so we must chose a safer set of bits to
 		// begin with.
-		permBits = 0700
+		permBits = 0o700
 	}
 	fs := f.Filesystem(nil)
 	err := fs.Mkdir(DefaultMarkerName, permBits)
@@ -113,7 +114,11 @@ func (f *FolderConfiguration) CreateMarker() error {
 
 // CheckPath returns nil if the folder root exists and contains the marker file
 func (f *FolderConfiguration) CheckPath() error {
-	fi, err := f.Filesystem(nil).Stat(".")
+	return f.checkFilesystemPath(f.Filesystem(nil), ".")
+}
+
+func (f *FolderConfiguration) checkFilesystemPath(ffs fs.Filesystem, path string) error {
+	fi, err := ffs.Stat(path)
 	if err != nil {
 		if !fs.IsNotExist(err) {
 			return err
@@ -131,7 +136,7 @@ func (f *FolderConfiguration) CheckPath() error {
 		return ErrPathNotDirectory
 	}
 
-	_, err = f.Filesystem(nil).Stat(f.MarkerName)
+	_, err = ffs.Stat(filepath.Join(path, f.MarkerName))
 	if err != nil {
 		if !fs.IsNotExist(err) {
 			return err
@@ -145,11 +150,11 @@ func (f *FolderConfiguration) CheckPath() error {
 func (f *FolderConfiguration) CreateRoot() (err error) {
 	// Directory permission bits. Will be filtered down to something
 	// sane by umask on Unixes.
-	permBits := fs.FileMode(0777)
+	permBits := fs.FileMode(0o777)
 	if build.IsWindows {
 		// Windows has no umask so we must chose a safer set of bits to
 		// begin with.
-		permBits = 0700
+		permBits = 0o700
 	}
 
 	filesystem := f.Filesystem(nil)