Make jenkins.sh work outside of Jenkins

Make the shared Bundler path conditional based on `$JOB_NAME` (which Jenkins
exports) so that the script can be used to run all tests on your own
machine. Strictly the name is wrong, if you can run it outside of Jenkins,
but it follows the convention we've used elsewhere.

NB: The hyphen after the variable name ensures that we don't bomb out on an
unbound variable when not present.
Replace old benchmark suite with new test docs

I don't think we've actually run these original benchmark tests in a while.
Nonetheless they have now been superseded by `spec/performance_spec.rb`,
apart from the handling of a large routing table, which will be taken care
of by story [#60237076].

Replace these with documentation about the tests we now have. So that
newcomers to the project know what's available, where to find it, and why.
Merge pull request #59 from alphagov/test_docs

[#60322580] Test documentation