tests: Increase stack size when nRF DK serial workaround is enabled

... and tests are built with no optimizations. The standard size may
not be sufficient in such circumstances.

Commit 4b110354 addressed the same
issue in a single test and used a higher value (1024) that turns out
to be not necessary, so revert also that change to make the cleanup
easier when the workaround will no longer be needed.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>