run-command tests: test stdout of run_command_parallel()

run-command tests: test stdout of run_command_parallel()

Extend the tests added in c553c72eed6 (run-command: add an
asynchronous parallel child processor, 2015-12-15) to test stdout in
addition to stderr.

When the "ungroup" feature was added in fd3aaf53f71 (run-command: add
an "ungroup" option to run_process_parallel(), 2022-06-07) its tests
were made to test both the stdout and stderr, but these existing tests
were left alone. Let's also exhaustively test our expected output
here.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>