Skip some online tests when in `./gradlew test --offline` mode

[HenkPoley]

Context: Bumped into issues running the tests in OpenAI Codex' offline VM. Skips publishPlugin and runPluginVerifier in --ofline mode. Probably useful in some other context as well.

Otherwise, you can turn on internet access of the VM. Testing works with the 'GET, HEAD and OPTIONS' HTTP-method filter enabled.

For reference, here is the 'environment' (VM) configuration script I'm using. Upgrades Gradle to 8.4 so it runs under Java 21 that they include. Also runs a bunch of commands that should retrieve and cache all dependencies.

#!/usr/bin/env bash

set -euo pipefail

# Update Gradle wrapper to version 8.4
sed -i.bak 's|distributionUrl=.*gradle-[^/]*-all.zip|distributionUrl=https\://services.gradle.org/distributions/gradle-8.4-all.zip|' ./gradle/wrapper/gradle-wrapper.properties

# Refresh dependencies (probably duplicates efforts)
./gradlew compileKotlin
./gradlew dependencies --refresh-dependencies
./gradlew test --refresh-dependencies
# ./gradlew test --offline
# ./gradlew buildPlugin
./gradlew help --refresh-dependencies --no-daemon

# Add note about offline testing to AGENTS.md
echo -e '\nIf you have no internet, you have to use `./gradlew test --offline` to run the tests.' >> AGENTS.md