Refactor out test_disk_usage_eviction tenant creation and add a custom
case with 4 tenants, 3 made with pgbench scale=1 and 1 made with pgbench
scale=4.
Because the tenants are created in order of scales [1, 1, 1, 4] this is
simple enough to demonstrate the problem with using absolute access
times, because on a disk usage based eviction run we will
disproportionally target the *first* scale=1 tenant(s), and the later
larger tenant does not lose anything.
This test is not enough to show the difference between `relative_equal`
and `relative_spare` (the fudge factor); much larger scale will be
needed for "the large tenant", but that will make debug mode tests
slower.
Cc: #5304