test: install PyYAML so the starlark route tests can load the plugin (#546)

Tests has been red on main since #535. All 13 failures in
test/web_interface/test_starlark_pixlet_routes.py are the same
ModuleNotFoundError: No module named 'yaml'.

The test loads plugin-repos/starlark-apps/tronbyte_repository.py by path --
deliberately, "the way the blueprint does", since the core web blueprint
really does exec that plugin module -- and the plugin imports yaml.

Nothing is undeclared. The plugin's own requirements.txt already pins
PyYAML>=6.0.2, and on a real rig the plugin store installs it. CI installs
only requirements.txt and requirements-test.txt, so a core test that reaches
into a plugin gets none of the plugin's dependencies.

PyYAML goes in the test requirements rather than the core ones because it is
not a core dependency: nothing in src/ or web_interface/ imports yaml. This is
the same shape as the psutil entry directly above it -- a package the core does
not require, installed so a test can exercise a real path instead of a stub.

Verified locally: with yaml available the file goes from 13 failures to 64
passing. (One unrelated failure remains on Windows only, where os.geteuid does
not exist.)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

requirements-test.txt

@@ -7,3 +7,8 @@ freezegun>=1.2,<2        # deterministic time for golden-image tests
 psutil>=6.0.0,<8.0.0     # optional at runtime; installed for tests so the
                          # /system/status endpoint's real path is exercised
 mypy>=1.5.0,<2.0.0       # static type checking (also pinned in .pre-commit-config.yaml)
+PyYAML>=6.0.2,<7.0.0     # not a core dependency: test_starlark_pixlet_routes loads
+                         # plugin-repos/starlark-apps/tronbyte_repository.py the way
+                         # the blueprint does, and that plugin imports yaml. The
+                         # plugin declares it in its own requirements.txt, which the
+                         # store installs on a real rig but CI never does.