fstest: Make Copy mutation test work properly

Before this change it could miss a mutation if the Modtime was cached

fstest/fstests/fstests.go

@@ -1276,6 +1276,8 @@ func Run(t *testing.T, opt *Opt) {
 				err = dst.SetModTime(ctx, fstest.Time("2004-03-03T04:05:06.499999999Z"))
 				if err != fs.ErrorCantSetModTimeWithoutDelete && err != fs.ErrorCantSetModTime {
 					assert.NoError(t, err)
+					// Re-read the source and check its modtime
+					src = fstest.NewObject(ctx, t, f, src.Remote())
 					assert.False(t, src.ModTime(ctx).Equal(dst.ModTime(ctx)), "mutating dst should not mutate src -- is it Copying by pointer?")
 				}