Add 'test_repeatedly', for help with flakey tests

[joshpencheon]

Issue Summary

Often, when investigating whether a test is flakey, it's helpful to be able to run a test multiple times. This tends to get done by either using a bash loop (which can be slow) or hacking about in the test file (which can get messy).

Using a bash loop to repeatedly instantiate a runner is slow:

for i in `seq 1 100`; do bin/rails test path/to/test.rb:123; done

And mucking around duplicating the test gets messy:

+ 100.times do |x|
- test 'something flakey' do
-   # ...
- end
+   test "something flakey - #{x}" do
+     # ...
+   end
+ end

This PR

This PR adds the test_repeatedly construct, to make the latter pattern (which executes more efficiently) quick and simple to do, whilst retaining some configurability.

- test 'something flakey' do
+ test_repeatedly 'something flakey' do
    # 
  end

[ollietulloch]

Thanks @joshpencheon - I've pointed an app at this branch locally and oberved an integration test running repeatedly for a given number of times, or 100 times if not specified.