The use testing.T.TempDir() seems to cause test failures
in CI environvements in those cases where temporary
directories' subdirs are created with permissions that
are different from the defaults used by testing.T.TempDir().
The snapshots package's tests seem to be heavily affected,
thus this replaces testing.T.TempDir() occurrences with
ioutil.TempDir() calls.

Related upstream issue:
- golang/go#40853