riotctrl.shell: flush pexpect before prompt

[miri64]

For some boards there is a weird syncing error happening: The output of a command issued via a ShellInteraction comes after the next command command issued. Examples of this can be seen in this action or in this comment. With flushing pexpected completely before the prompt, this does not happen again. Thanks @fjmolinas for the hint!

For testing the tests here should pass, as well as tests/congure_test in RIOT on the boards we found the issue for; namely nucleo-f411re, b-l072z-lrwan1, and b-l475e-iot01a:

$ git fetch upstream refs/pull/21/head # assuming you are in the riotctrl repo and RIOT-OS/riotctrl remote is called upstream
$ git checkout FETCH_HEAD
$ pip install .[rapidjson] --upgrade
$ cd ../RIOT     # assuming this is where your RIOT repo is
$ git checkout master
$ git pull
$ BOARD="<one of the above>" make -C tests/congure/test -f flash test

[miri64]

I also made some sample tests with the release tests. They also seem to still work.

[fjmolinas]

Suggested change

	# flush pexpect up to 10000 characters to fix syncing issues with
	# flush pexpect up to 10000(arbitrarily large value) characters to start with an empty buffer.
	# This fixes issues where REPLWrapper would capture previous commands output.

Maybe the comment can be a little more detailed? Feel free to modify of course

[miri64]

Done.

[fjmolinas]

ACK, please squash

[miri64]

Squashed.

[miri64]

Squashed and fixed issue pointed out by the CI.

[fjmolinas]

RE-ack

[miri64]

Thanks!