sync: fix testLoggerVsLsf when backend only reads modtime

There are some backends (like PikPak) that advertise a precision of
fs.ModTimeNotSupported but do actually return a modtime when asked. In the case
of PikPak, it is because the modtime can be read but not written, and is not
considered reliable enough to use for syncing.

Before this change, testLoggerVsLsf got confused in this scenario (expected a
blank modtime but got non-blank). Adding to the confusion, it only reaches this
code if the backend happens to support md5 hashes, and the fsrc and fdst have
the same precision.

This change fixes the issue by setting the modtime string on both sides to
"none" in this scenario. Note that we can't use "" (blank) because
(operations.ListFormat).AddModTime would replace that with "2006-01-02 15:04:05".

fs/sync/sync_test.go

@@ -3031,6 +3031,9 @@ func DstLsf(ctx context.Context, Fremote fs.Fs) *bytes.Buffer {
 
 	list.SetSeparator(";")
 	timeFormat := operations.FormatForLSFPrecision(Fremote.Precision())
+	if Fremote.Precision() == fs.ModTimeNotSupported {
+		timeFormat = "none"
+	}
 	list.AddModTime(timeFormat)
 	list.AddHash(hash.MD5)
 	list.AddSize()
@@ -3082,7 +3085,7 @@ func testLoggerVsLsf(ctx context.Context, fdst, fsrc fs.Fs, logger *bytes.Buffer
 			elements := bytes.Split(line, []byte(";"))
 			if len(elements) >= 2 {
 				if !canTestModtime {
-					elements[0] = []byte("")
+					elements[0] = []byte("none")
 				}
 				if !canTestHash {
 					elements[1] = []byte("")