[nrf noup] sample/smp_svr: nrf54l15dk test cases

[michalek-no]

fixup! [nrf noup] samples/smp_svr: nrf54l15pdk ext-flash partition

Added nrf54l15dk testcases as copies of nrf54l15pdk.

backport of #2167

moved from #2168

[rlubos]

@michalek-no No need to backport sdk-zephyr PRs manually, just add the backport label in the original PR, it will be backported automatically when merged.

[michalek-no]

@michalek-no No need to backport sdk-zephyr PRs manually, just add the backport label in the original PR, it will be backported automatically when merged.

no way to update #2168 due to single origin branch. And there is a mess as you can see.

[rlubos]

@michalek-no No need to backport sdk-zephyr PRs manually, just add the backport label in the original PR, it will be backported automatically when merged.

no way to update #2168 due to single origin branch. And there is a mess as you can see.

I'm not asking to modify #2168, I'm asking to add backport label to #2167 instead of creating PRs manually at all. It's the cleanest and most manageable way to handle those sdk-zephyr PRs towards release branch.