Allow to configure integration specific requires.

[mbj]

Currently mutant and minitest integration assume certain files to exist when pre-loading the tests.

On more advanced setups the tests will not be in standad locations for various reasons. Hence we should add something like --require-integration that allows to manually specify the test files to require.

Note: Tests are intentionally loaded AFTER the subject matching. This speeds up subject matching significantly as especially rspec trashes the object space with Class instances whos #name take ages to evaluate.

Also mutant tries to encourage having a main require entrypoint that loads the app "in full" before the tests loading trigges autoloads. This also improves performance down the line as autoloads are likely to be expaned already when kills are executed.