t5534: split stdout and stderr redirection

t5534: split stdout and stderr redirection

On atomic pushing failure with GnuPG, we expect a very specific output
in stdout due to `--porcelain` switch.

On such failure, we also write down some helpful hint into stderr
in order to help user understand what happens and how to continue from
those failures.

On a lot of system, those hint (in stderr) will be flushed first,
then those messages in stdout will be flushed. In such systems, the
current test code is fine as is.

However, we don't have such guarantee, (at least) there're some real
systems that writes those stream interleaved. On such systems, we may
see the stderr stream written in the middle of stdout stream.

Let's split those stream redirection. By splitting those stream,
the output stream will contain exactly what we want to compare,
thus, saving us a "sed" invocation.

While we're at it, change the `test_i18ncmp` to `test_cmp` because we
will never translate those messages (because of `--porcelain`).

Signed-off-by: Đoàn Trần Công Danh <redacted>
Signed-off-by: Junio C Hamano <redacted>