test: tolerate OutPayload/InTrailer reorder in RetryStats (#9272)

Fixes #5881

`Test/RetryStats` flakes about 1 in 10k runs. The test already tolerates
a payload/trailer race for the final attempt (positions 13 and 14),
where the InPayload and InTrailer stats can arrive out of order. The
same race applies to the transparent retry attempt: its OutPayload and
InTrailer stats (positions 6 and 7) can also be recorded out of order,
which the test does not yet tolerate. The reported failure is `at
position 6: got *stats.InTrailer; want *stats.OutPayload`.

This extends the existing tolerance to the retry attempt's
OutPayload/InTrailer pair, using the same swap technique already applied
at positions 13 and 14.

RELEASE NOTES: N/A