docs: verify and update how-to/testing for current CI (GitHub Actions + DLS GitLab)

[coretl]

Target page

how-to/testing

The page is a full stub — the source (reference/testing.rst) describes a
Travis CI setup that has since moved to GitHub Actions (simulation tests) and
DLS internal GitLab (full FPGA build tests).

Category: verify-then-write

What a human must do (~3 hours)

1. Verify the current CI arrangement:
   • Which test suite(s) run on **GitHub Actions** (open to contributors)?
     Typically the Python / cocotb simulation tests.
   • Which test suite(s) run on **DLS internal GitLab** (requires DLS access)?
     Typically the full FPGA bitstream build and hardware-in-the-loop tests.
2. For each suite: document the trigger (PR/push/manual), the command to run
   locally, and any required setup (Python version, simulator, DLS VPN).
3. Note which tests require access to real hardware vs simulation only.
4. Replace the stub with the verified content.

Post as a PR against docs/how-to/testing.md on the docs branch.

Source

docs/_legacy_rst/reference/testing.rst — 274 words (Python tests, HDL tests).