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/fs/filesystem.go

@@ -172,21 +172,23 @@ var (
 
 // Equivalents from os package.
 
-const ModePerm = FileMode(os.ModePerm)
-const ModeSetgid = FileMode(os.ModeSetgid)
-const ModeSetuid = FileMode(os.ModeSetuid)
-const ModeSticky = FileMode(os.ModeSticky)
-const ModeSymlink = FileMode(os.ModeSymlink)
-const ModeType = FileMode(os.ModeType)
-const PathSeparator = os.PathSeparator
-const OptAppend = os.O_APPEND
-const OptCreate = os.O_CREATE
-const OptExclusive = os.O_EXCL
-const OptReadOnly = os.O_RDONLY
-const OptReadWrite = os.O_RDWR
-const OptSync = os.O_SYNC
-const OptTruncate = os.O_TRUNC
-const OptWriteOnly = os.O_WRONLY
+const (
+	ModePerm      = FileMode(os.ModePerm)
+	ModeSetgid    = FileMode(os.ModeSetgid)
+	ModeSetuid    = FileMode(os.ModeSetuid)
+	ModeSticky    = FileMode(os.ModeSticky)
+	ModeSymlink   = FileMode(os.ModeSymlink)
+	ModeType      = FileMode(os.ModeType)
+	PathSeparator = os.PathSeparator
+	OptAppend     = os.O_APPEND
+	OptCreate     = os.O_CREATE
+	OptExclusive  = os.O_EXCL
+	OptReadOnly   = os.O_RDONLY
+	OptReadWrite  = os.O_RDWR
+	OptSync       = os.O_SYNC
+	OptTruncate   = os.O_TRUNC
+	OptWriteOnly  = os.O_WRONLY
+)
 
 // SkipDir is used as a return value from WalkFuncs to indicate that
 // the directory named in the call is to be skipped. It is not returned
@@ -354,3 +356,20 @@ func unwrapFilesystem(fs Filesystem, wrapperType filesystemWrapperType) (Filesys
 		}
 	}
 }
+
+// WriteFile writes data to the named file, creating it if necessary.
+// If the file does not exist, WriteFile creates it with permissions perm (before umask);
+// otherwise WriteFile truncates it before writing, without changing permissions.
+// Since Writefile requires multiple system calls to complete, a failure mid-operation
+// can leave the file in a partially written state.
+func WriteFile(fs Filesystem, name string, data []byte, perm FileMode) error {
+	f, err := fs.OpenFile(name, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+	_, err = f.Write(data)
+	if err1 := f.Close(); err1 != nil && err == nil {
+		err = err1
+	}
+	return err
+}