Attempt to deflake DBTestXactLogIterator.TransactionLogIteratorCorruptedLog (#8627)

Attempt to deflake DBTestXactLogIterator.TransactionLogIteratorCorruptedLog (#8627)

Summary:
The patch attempts to deflake `DBTestXactLogIterator.TransactionLogIteratorCorruptedLog`
by disabling file deletions while retrieving the list of WAL files and truncating the first WAL file.
This is to prevent the `PurgeObsoleteFiles` call triggered by `GetSortedWalFiles` from
invalidating the result of `GetSortedWalFiles`. The patch also cleans up the test case a bit
and changes it to using `test::TruncateFile` instead of calling the `truncate` syscall directly.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8627

Test Plan: `make check`

Reviewed By: akankshamahajan15

Differential Revision: D30147002

Pulled By: ltamasi

fbshipit-source-id: db11072a4ad8900a2f859cb5294e22b1888c23f6