Add '-insecureTLS' option, to disable cert checks

We are generally going to be talking to https URLs, but
running on hostnames that do not map to their certificates.

Hence, CA and CN checks are going to fail - allow overiding
Add a 'Origin Probe Busting' test

Certainly with Fastly, it takes a short (~20s) amount of time to
recognise that our 'Origin' is listening and responding to requests.

This test effectively polls the CDN to wait for the origin probe to
succeed. We can then move on to our actual tests.

NB: We need to wait *before* breaking from the attempt loop, to
    ensure that all the CDN nodes have passed their probe tests.
    This may need tweaking, though 5s seems to be enough time for
    this to happen
Test we are receiving from Origin

Set up a new request handler that only responds to a random UUID.

Serve this request once - and confirm that it has been received.
Reduce logic needed to handle insecureTLS
Move 'origin successfully probed' test into Init

We require that Origin has been marked as available
for all other tests, so this should be an init helper
test rather than a regular test.

This also ensures that this will be run if we limit the
tests via -run
Remove unneccessary URL fragment
make use of %q for quoted strings
Make check-origin-is-up loop more readable

* remove the use of a magic number for sleep duration
* break out quickly if maxRetries hit
* explicit comment as to why we are sleeping before break
  in success case.
Pass edgeHost as string to testOriginIsEnabled

There isn't a need to pass edgeHost as a pointer, so
lets not bother. It's more readable as a string.
Properly move origin-enabled check into init()

As a result, remove the `testing` struct, and make it
a function that potentially returns an error which we
then respond to in init().
Merge pull request #6 from alphagov/origin_by_default

Test for 'Origin by default', plus supporting tests & flags
Add complete test for cached responses

Ensure that the CDN is behaving as CDNs should. Response to first request is
cached and delivered on subsequent requests without hitting origin.