test(upgrade): run channel flow tests against the mock server offline

[bartlomieju]

The stable, canary, specific_stable and specific_canary upgrade
spec tests each copied the binary and ran a real deno upgrade, hitting
GitHub and dl.deno.land and downloading a full release archive on every
CI run across every platform. That burns bandwidth and couples PR CI to
the live release state: when a new version ships mid-flight, the output
match can break on unrelated PRs (which is exactly what happened when
v2.9.4 was published).

This points these four tests at the existing test-server mock instead,
the same mechanism delta and checksum already use, via
DENO_TESTING_UPGRADE and DENO_DONT_USE_INTERNAL_BASE_UPGRADE_URL. The
version lookup and the download both resolve to localhost:4545, so the
tests are fully offline, deterministic (fixed sentinel versions), and run
in tens of milliseconds instead of ~13s each.

The mock serves a placeholder, non-executable binary, so each upgrade
runs through version resolution and the download and then fails
validation. The tests assert the resolved version and the mocked
download URL rather than a completed install, matching the pattern the
existing mocked upgrade tests use.

Canary downloads previously had no test hook, so this extends the
DENO_TESTING_UPGRADE redirect to the canary download URL in
get_download_url (only active when that env var is set, so no
production impact) and teaches the test server to serve zips under
/deno-upgrade/canary/, mirroring the existing /deno-upgrade/download/
handling.

The two end-to-end tests that need a real runnable binary to assert the
upgraded executable actually runs and reports the expected version
(space_in_tmp, out) are intentionally left on the real network for
now; moving those behind a mock needs a runnable cross-platform stub or a
scheduled real-network job, which is a follow-up.