github/workflows: add scheduled CI runs

Test results may vary due to new dependency versions. Daily CI runs
should help us find issues early.

This becomes especially important once all of labgrid's dependencies get
unpinned.

Do not enable scheduled runs for the stable branch just yet, because our
switch to pyproject.toml makes the current workflow incompatible with
the stable branch. We can add the stable branch once we have a new
release. Future incompatibilities between stable and master should not
be a big issue: we can release a new version after incompatible changes
(since we do not maintain previous stable branches) or cherry-pick
self-contained changes to the CI config to stable.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

.github/workflows/scheduled-unit-tests.yml

@@ -0,0 +1,27 @@
+name: scheduled unit tests
+
+on:
+  schedule:
+    - cron: '10 8 * * *'
+
+jobs:
+  scheduled-unit-tests:
+    name: scheduled unit tests
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11']
+        branch: ['master']
+    uses: ./.github/workflows/reusable-unit-tests.yml
+    with:
+      python-version: ${{ matrix.python-version }}
+      branch: ${{ matrix.branch }}
+  scheduled-unit-tests-docker:
+    name: scheduled docker tests
+    strategy:
+      fail-fast: false
+      matrix:
+        branch: ['master']
+    uses: ./.github/workflows/reusable-unit-tests-docker.yml
+    with:
+      branch: ${{ matrix.branch }}