Window functions: frame-adhering aggregate without ORDER BY clause

Hi Listers,

among the window tests (src/test/regress/expected/window.out), I noticed
the presence of tests that rely upon the order of rows not determined by
any ORDER BY clause, such as:

SELECT sum(unique1) over (rows between 2 preceding and 2 following
exclude no others),
    unique1, four
FROM tenk1 WHERE unique1 < 10;

Expected result:

 sum | unique1 | four
-------+-----------+------
      7 |            4 |    0
    13 |            2 |    2
    22 |            1 |    1
    26 |            6 |    2
    29 |            9 |    1
    31 |            8 |    0
    32 |            5 |    1
    23 |            3 |    3
    15 |            7 |    3
    10 |            0 |    0
(10 rows)

The current row's frame and, consequently, the result of the sum
aggregate depend on the order produced by the sequential scan of table
tenk1. Since such order is, in general, not part of PG's defined
behavior, what purpose do the tests that rely upon it serve?

Following up to that, how is an EXCLUDE GROUP defined to behave in
absence of any ORDER BY clause? It seems to exclude the entire window
frame according to this test:

SELECT sum(unique1) over (rows between 2 preceding and 2 following
exclude group),
    unique1, four
FROM tenk1 WHERE unique1 < 10;

Expected result:

 sum | unique1 | four
-------+-----------+------
        |            4 |    0
        |            2 |    2
        |            1 |    1
        |            6 |    2
        |            9 |    1
        |            8 |    0
        |            5 |    1
        |            3 |    3
        |            7 |    3
        |            0 |    0
(10 rows)

Thanks in advance and best regards,
Romain