Moves the require for puma/events from the individual test files and
into the main test helper.
The reason for this was while I was debugging the `test_puma_server.rb`
tests I was getting an error for an uninitialized constant for
`Puma::Event`.
Moving the require from the individual files to the test help means the
`puma/events` will always be included instead of having to remember to
do that. It makes debugging individual tests locally easier.
running `rake` loads all files starting with `test_` which is not supposed to also load the helper,
since the helper should be loaded by each test and is not a test itself.
