The motivation for this addition was to unit test the function `wallet.cpp:ExtractPubKey()` (see recent change in commit 798a589aff64b83a0844688a661f4bd987c3340c) which is however static and only indirectly available via the public methods `AddWatchOnly()`, `LoadWatchOnly()` and `RemoveWatchOnly()`. Since the first of those methods also stores the addresses to the disk, the second, simpler one was chosen which only operates in memory.

ninja check