tests: gate remote flatten fixtures as long tests

[HNO3Miracle]

TestFlatten_RemoteAbsolute fetches the CloudEvents v0.2 schema from raw.githubusercontent.com. Reuse the existing -enable-long test switch so the default unit suite works offline while preserving an explicit path to run the remote fixture coverage.

Tested with go test ./....

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.44%. Comparing base (f65b576) to head (dfb6db4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage   91.83%   91.44%   -0.39%     
==========================================
  Files          30       30              
  Lines        3393     4244     +851     
==========================================
+ Hits         3116     3881     +765     
- Misses        194      281      +87     
+ Partials       83       82       -1     

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[fredbi]

Instead of simply disabling the test, I'd prefer that the spec is served on an embedded httpttest server.

[fredbi]

Disabling the test is not desirable, and I disagree with weakening the test suite.

What is needed instead to really improve this test is to serve the entire spec locally (e.g. with a httptest server like you've seen in your "go-openapi/validate" PR). But the fixture needs some tweaking to resolve remotes against a localhost test server.