Test::Harness assumes that you want to run all of the .t files in the t/
directory in ascii-betical order during make test unless you say otherwise.
This leads to some interesting naming schemes for test files to get them in
the desired order. This interesting names ossify when they get into source
control, and get even more interesting as more tests show up.

Test::Manifest overrides the default behaviour by replacing the
test_via_harness target in the Makefile. Instead of running at the t/*.t
files in ascii-betical order, it looks in the t/test_manifest file to
find out which tests you want to run and the order in which you want
to run them. It constructs the right value for MakeMaker to do the right thing.