Added test cases for result.all intermixed output.

Added fixtures/noop-132 which should output the sequence '123' but due to the unpredictable nature of the stdout/stderr it outputs '132' as the most common cases
I've tried, as listed on noop-132, a different number of technique to make it predictable.

On execa.sync, as it's using spawnSync internally, I couldn't get the stream and merge them.

As such the output of result.all is just a concatenation of stdout + stderr.
This makes the result.all in different use case (ex. async vs. sync) unreliable for string comparison, but at least useful for logging or checking on the output.

… nothing

- execa.sync() cannot provide a true interleaved `all` stream because of
the implementation of child_process.spawnSync, which does not expose `stdout`
and `stderr` streams until termination. We could only concatenate the streams in the end.
- Does not create the mixed stream if both streams do not exist.

…ync()

The delay 6000ms is approximate, it may not work on every machine.

…132 (1000ms)

Testing with test.serial and a delay of only 1000ms seems to pass multiple
test runs consistently. Let's see if it passes on CI testing as well.