Fix test failure due to wrong module name

[c960657]

One test is consistently failing on master:

1) Error:
Clockwork::Manager#test_0012_exceptions are trapped and logged:
NameError: uninitialized constant MiniTest
Did you mean?  Minitest
    test/manager_test.rb:1[20](https://github.com/c960657/clockwork/actions/runs/10076662391/job/27857488923#step:4:21):in `block (2 levels) in <top (required)>'

Minitest was renamed to Minitest in v5. Ruby stdlib previously carried minitest v4.