Summary:
Fix an assert (so only applicable when the library is built in DIAGNOSTIC mode), that could race with the recently added locking code.

When splitting a page into its parent, in DIAGNOSTIC mode, WT_REF structures are asserted to be in WT_REF_SPLIT mode, and that isn't necessarily correct because the WT_REF is still discoverable, and can reasonably be in WT_REF_LOCKED mode.

Since the increase in cursor-walk WT_REF locking added in WT-5219, format runs are triggering the assert.