Restructure performance specs

So that we can add a context that doesn't use "two healthy backends".
Allow vegeta args to be passed to shared example

The VegetaHelper already allows a hash of additional CLI arguments to be
passed on. Allowing individual tests to modify the duration or rate of
requests. This reinstates that ability by allowing a hash to be passed to
the shared example, which defaults to `{}`.
[#59040160] Throughput and concurrency tests

Tests for throughput (requests per second) and concurrency (requests in
flight at a single point in time). These appear to be mostly limited by the
file descriptor ulimit of the process/shell that they are run from.

Because we're launching the following from rspec all in parallel:

- 2x Vegeta attack.
- 2x Vegeta report.
- 2x Simple backend.
- Router.

The combination of the above, not necessarily the router, is also pretty
harsh on system resources such as CPU. So it would be rude to run these from
CI. As such I'm marking them as optional. You can opt into running as
follows:
```
ulimit -n 4096
rspec --tag ulimits
```
Merge pull request #58 from alphagov/59040160-perf_bound_testing

[Fixes #59040160] Throughput and concurrency tests
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Nonetheless they have now been superseded by `spec/performance_spec.rb`,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