make_tmp_name() creates temporary name with prefix containing PID and
TID. This prefix influences max length of the rest of the name (the
whole name is limited by NAME_LEN). During the test run PID and TID
can change. PID increases when the server restarts. TID is incremented
with the new connections (generated by next_thread_id(), is not the
posix thread ID).

During the test run PID can increase max 2 decimal positions: from
tens to thousands this requires ~900 restarts. TID depends on
connection count, but for test we assume it will not trespass 100000
connections which is 5 decimal positions. So it should be enough to
reserve 7 characters for PID and TID increment.

The patch reserves more: it reserves 12 characters for 7-decimal PID
and 5-decimal TID plus 4 chars of additional reserve (for future
prefix changes) and assumes minimal legth of the prefix is 30 bytes:

#sql-backup-PID-TID-
#sql-create-PID-TID-

4-6-PID-TID- is 10 + 4 + PID + TID, so PID + TID is 16 chars (7 + 5 + 4).