Test that for X-Served-By is set by Edge

We check for the Fastly format of cache-server1234-LOC, which is going
to change between providers, but this at least starts that documentation
going.

Also, check for existance of this field.
Rename Test and regexp to highlight Fastly-specific

This test is designed to be fastly specific, the pattern expected
of X-Served-By is not likely to be followed by other CDN providers.

Rename the Test (so that we can do -run Fastly) and regexp var name
accordingly.
X-Served-By test does not need to talk to origin

We are not testing origin-specific behaviour, just that the
provider sets an X-Served-By header matching our expectations.

This means that this test can now run against a live environment.
Merge branch 'master' into served_by_node_id
Mock Fastly X-Served-By behaviour in VCL
Merge pull request #11 from alphagov/served_by_node_id

Test that for X-Served-By is set by Edge