Reuse the test runner server across requests

[piraz]

Problem

testrunner.HttpTestRunner creates and closes a new httptest.Server for every request. This prevents multi-request flows from preserving server lifecycle state and makes the runner unsuitable for persistent HTTP sessions.

Scope

• Reuse one httptest.Server for the lifetime of an HttpTestRunner.
• Reuse one HTTP client so cookies and transport state persist across requests.
• Preserve dynamic WithHandler and WithHandlerFunc behavior.
• Close the server automatically when the owning test completes.

Acceptance criteria

• Multiple requests through the same runner use the same server.
• Session cookies survive between requests.
• Existing simple route and handshake tests continue to pass.
• The runner has regression coverage for persistent server/session behavior.