[Fixes #51] Allow failover tests to be skipped

Add a new `-skipFailover` flag that will prevent the additional "backup"
backends from being setup and skips all tests in cdn_failover_test.

I did start an implementation a while ago whereby you could specify a
variable number of backends, but I don't think it's that useful right now.
This is the most efficient thing to do.
Raise an error on slow requests

Slow requests can cause our tests to fail in unexpected ways. So we should
raise errors on them alone.

NB: requestTimeout is a byte-to-byte timeout so it won't be triggered if a
request takes 10s but is still pushing data very slowly.
Merge pull request #73 from alphagov/51-skip_failover_tests

[Fixes #51] Allow failover tests to be skipped
Merge pull request #74 from alphagov/error_on_slow_requests

Raise an error on slow requests
Merge branch 'master' into 7-tls_backends

Conflicts:
	common_setup_test.go