Split test workflow into PR and main branch push

[jovnc]

Separates the single test.yml workflow (which used two jobs with if: guards on github.event_name) into two dedicated files:

1. test-pr.yml: runs on pull_request_target with the e2e-test environment gate to protect secrets from untrusted PR code
2. test.yml: runs on push to main and workflow_dispatch

This makes the intent of each workflow clearer and removes the need for runtime if: conditions to differentiate between event types.