Contribution description

This is an attempt to fix #12214 and it does 2 things:

• cleanup of the test script
• add a 1s delay after the shell is effectively ready (> displayed) before requesting the interface with the ifconfig command.

After this fix, the test is working reliably locally.

For me this is a bit minor, so I don't really think it needs a backport but maybe other will disagree.

Testing procedure

Use the following script (from the RIOT base directory):

#! /bin/bash -xe

make --no-print-directory -C tests/nordic_softdevice/

for i in {0..10}; do
    make --no-print-directory -C tests/nordic_softdevice/ flash-only test
done

On master, it fails most of the time. With this PR it works all the time.

Issues/PRs references

fixes #12214