arrow

GH-15141: [C++] fix for unstable test due to unstable sort (#15142)

The sorting done by orderby is not stable.  This means, given the input:

a | b
--- | ---
1 | false
1 | true

the test could have generated both `[false, true]` and `[true, false]` for the `b` column.  We likely did not encounter this before https://github.com/apache/arrow/commit/498b645e1d09306bf5399a9a019a5caa99513815 because the entire thing was run serially (even though there was a `parallel` option it was not setup correctly).

Now that things are properly running parallel the results are non-deterministic.  We could remove the `b` column but I feel it is a better stress test to have at least one payload column.  So I changed the test to only compare the key array and not the payload array.
* Closes: #15141

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>