Some unit tests weren’t being run on iOS

Configuration glitches made a few tests run on Mac only, not iOS. Fixed.
Delete sqlite -wal and -shm files along with rest of database

-[CBLDatabase deleteDatabase:] should clean up the side files created
by SQLite. (I hate hardcoding their names, but I don't know any
alternative.)
Better creation of empty databases in unit tests

Added private debug-build-only CBLManager method -createEmptyDatabase:,
for use in API unit tests.