tests/gnrc_dhcpv6_client: fix expected output

[miri64]

Contribution description

The output of the shell command nib prefix show was changed in #12776 by adding a space between the number of seconds and the unit. Due to this the test tests/gnrc_dhcpv6_client now fails as it expects the output to be without spaces.

As the new riotctrl_shell "accidentally" accounted for that "error", I decided to rather fix the test than reverting the addition of the whitespace. There is no other test in master that checks this output.

Testing procedure

The test requires kea to be installed. It is available in most Linux distributions as a package, but many don't create the status directory of the server:

sudo mkdir -p /var/kea/

Then run on a board of your choice (that supports ethos):

make -C tests/gnrc_dhcpv6_client flash
sudo make -C tests/gnrc_dhcpv6_client test

Issues/PRs references

Fixes a "regression" introduced in #12776

[benpicco]

Patch works on native, but the test is buggy:

• on native it is required to run dist/tools/tapsetup/tapsetup before the test works. This is not documented.
• on a board (e.g. samr21-xpro with ethos) the test does not work at all as kea-dhcp6.conf hard-codes tapbr0.

[miri64]

* on `native` it is required to run `dist/tools/tapsetup/tapsetup` before the test works. This is not documented.

Where should this be documented if not in the README (where it is documented) ;-)

[miri64]

* on a board (e.g. `samr21-xpro` with `ethos`) the test does not work at all as `kea-dhcp6.conf` hard-codes `tapbr0`.

Also documented in the test's README...

[miri64]

Thanks for testing and reviewing :-)

[miri64]

Backport provided in #14528