tests: Remove May15 test
This test is no longer relevant.
It was introduced in 8c222dca4f961ad13ec64d690134a40d09b20813 to check
the switch to 1MB blocks after the BDB too-many-locks issue back in
- The switching code has been long since removed.
It also needs a specific data file that is hard to find. I've verified
in #6320 that it still passes, however I think there is zero reason to
keep it.
Closes #6320.