tests: make first_run.v work with current V and on Windows

[medvednikov]

What

Makes tests/first_run.v compile with the current V compiler and run on Windows (in addition to macOS/Linux), so it can be used as an end-to-end "gitly serves the expected HTML" test — including from vlang/v's Windows CI.

Why

The test had bit-rotted and no longer ran:

• It used the generic json.decode[T](body) or { … } form, but imported json (vlib/json, whose API is json.decode(T, s)). With current V this fails to compile. Fixed by import x.json2 as json (the convention already used elsewhere in gitly: github.v, webhook.v).
• It compiled gitly without -d sqlite, so gitly used its default PostgreSQL backend and panicked at startup (Connection to a PG database failed). Fixed by compiling with -d sqlite (embedded DB, no external service needed).
• It started gitly with os.execute('./gitly.exe &') and stopped it with pkill -9 — both Unix-only. On Windows the server never backgrounded/stopped, and after() then failed to delete the still-locked sqlite file. Fixed by using os.new_process for a cross-platform start + signal_kill/wait stop (DB/repos are removed only after the process exits).
• -d use_libbacktrace / -d use_openssl are skipped on Windows (libbacktrace isn't available there; the native schannel https backend is used instead).

Reliability knob

GITLY_TEST_SKIP_REMOTE_REPO=1 skips the tests that clone a remote GitHub repo (network + outbound git/https heavy — the flakiest part). With it set, only the pure HTML/serving checks run (index, register, user page, login, static CSS, oauth, local repo creation). Default behavior (full test) is unchanged.

Testing

Validated locally on macOS with the current vlang/v:

• GITLY_TEST_SKIP_REMOTE_REPO=1 v -g run tests/first_run.v → all tests passed, clean shutdown, no leftover process/port.
• Full run reaches and passes every HTML/serving assertion; only the remote-clone step depends on the environment.

Paired with a new Windows job in vlang/v that runs this (smoke mode) to verify gitly (which serves via veb → fasthttp) works on Windows.