github/workflows: Add CI to run tests against zephyr with qemu.

With this change, the zephyr port is tested against the standard test suite
via the following scheme:
- the zephyr port is built with the `qemu_cortex_m3` board and the
  `prj_minimal.conf` configuration
- `qemu-system-arm` runs `zephyr.elf`
- the zephyr console is redirected to a pts/pty
- `tests/run-tests.py` is run in bare-metal mode against the pts/pty device

This allows testing the zephyr port as though it were a physical board
attached over a serial port.

Signed-off-by: Damien George <damien@micropython.org>

.github/workflows/ports_zephyr.yml

@@ -27,3 +27,8 @@ jobs:
       run: source tools/ci.sh && ci_zephyr_install
     - name: Build
       run: source tools/ci.sh && ci_zephyr_build
+    - name: Run main test suite
+      run: source tools/ci.sh && ci_zephyr_run_tests
+    - name: Print failures
+      if: failure()
+      run: tests/run-tests.py --print-failures