At this point it is a bit tricky to measure the performance increase
from the new message deletion code, since the same commit (9ad19e4)
that improved the performance also seems to have fixed a bug with an
uncaught Xapian exception triggered by this test.
I couldn't run the performance tests on my machines due to a hardcoded
bash path. Use env for finding bash in weird systems like NixOS.
Signed-off-by: William Casarin <jb55@jb55.com>
