test/osd: cover clear_omap tombstoning of cached keys

Add a regression test that processes an insert so the keys are cached
in key_map, then processes a later clear_omap and asserts the cached
keys are returned as tombstones.

This would have caught the by-value bind in process_entries, where
clear_omap mutated copies and left already-cached keys untouched.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>