Test to validate re-reading files that were closed with active history

With WT-6819 the files that have active history, ie content newer than the oldest timestamp, can be closed when idle. We should add more testing around reading such files under a long-running transaction.

Opening files reset their write generation numbers. The write generation numbers are also used for visibility checks. The test should make sure that under a long-running transaction if a file gets closed and then reopened later, we continue to read the correct version of the data.

The relevant code path will involve run_write_gen or the "runtime write generation". The idea is that when a dhandle is re-opened, it should get the same base write generation. Any transaction IDs written before getting the dhandle got closed, should NOT be wiped since we didn't actually restart the system.