Fix formatting error in TestRespHeaderAge

This should use Fatalf instead of Fatal:

    cdn_resp_headers_test.go:48: possible formatting directive in Fatal call
Fix "Println call ends with newline"

Replace it with a Printf that contains two newlines to fix:

    common_setup_test.go:41: Println call ends with newline
Formatting errors in TestHelpersCDNServeStop

These were using Error instead of Errorf:

    helpers_test.go:58: possible formatting directive in Error call
    helpers_test.go:78: possible formatting directive in Error call

But in changing them to Errorf I also found that a boolean can't be printed
as `%q`. It should use `%t` instead.

I've also moved the expected value to a variable so that we don't repeat it
and it makes it more obvious why we're doing a comparison instead of just
`started` or `!started`.
Merge pull request #64 from alphagov/fix_go_vet_warnings

Fix various go vet warnings