test(cache): drop test server handling that tests never exercise

[wan9chi]

Motivation

The remote cache test server from #754 handles cases the tests never hit: signals sent to the wrapper, a missing command, spawn failures, non-numeric blob IDs, and cbor-http calls with an absolute URL or without VP_REMOTE_CACHE_URL. Test infra should only handle what the tests exercise.

remote-cache-server now starts the server, runs the command, closes the server, and exits with the command's exit code. The blob route and state loading check whether the file exists. cbor-http always appends its path to VP_REMOTE_CACHE_URL. The command_wrapper case goes away; the other snapshots are unchanged.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -0.28%  [ -8.28% ..  +6.54%]  overhead  +261.83%
dynamic/access             change  -0.15%  [ -1.22% ..  +0.79%]  overhead   +13.63%
dynamic/access-relative    change  -0.12%  [ -1.00% ..  +1.24%]  overhead   +60.07%
dynamic/access-contended   change  -0.01%  [ -3.03% ..  +1.58%]  overhead   +15.06%
static/launch              change  +0.18%  [ -5.34% ..  +5.05%]  overhead  +717.97%
static/access              change  -0.11%  [ -1.43% ..  +0.90%]  overhead  +808.53%
static/access-relative     change  -0.29%  [ -1.41% ..  +0.67%]  overhead +1378.21%
static/access-contended    change  -0.70%  [ -1.52% ..  +0.73%]  overhead +3192.14%

macos

dynamic/launch             change  -0.07%  [ -4.93% ..  +4.13%]  overhead  +239.97%
dynamic/access             change  +0.69%  [-22.58% ..  +8.24%]  overhead    +3.38%
dynamic/access-relative    change  -1.29%  [-28.86% ..  +6.02%]  overhead  +245.07%
dynamic/access-contended   change  +1.50%  [-19.68% ..  +9.90%]  overhead    +0.42%

windows

dynamic/launch             change  -0.30%  [ -4.58% ..  +4.44%]  overhead   +26.27%
dynamic/access             change  +0.00%  [ -5.16% ..  +2.20%]  overhead    +1.11%
dynamic/access-relative    change  +0.53%  [ -1.64% ..  +4.33%]  overhead    +1.80%
dynamic/access-contended   change  +0.18%  [ -2.41% .. +28.21%]  overhead    +1.97%