Make test:isolated run without bundler for Action Mailer

Action Mailer tests weren't able to run in isolation without the bundle
exec prefix since we were requiring gems before requiring abstract_unit.

We don't need the `gem` call thus and the require_relative since the
test directory should be present in the load path when we run any test.
Merge pull request #16741 from robin850/am-isolated-wo-bundle

Make test:isolated run without bundler for Action Mailer