GITHUB: add workflows for testing

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/.github/workflows/testing.yml

+# This Source Code Form is licensed MPL-2.0: http://mozilla.org/MPL/2.0
+
+# https://rhysd.github.io/actionlint/
+
+name: Testing
+on: [push]
+jobs:
+  linux:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Test Build
+      run: misc/dbuild.sh
+  macos:
+    runs-on: macos-11
+    steps:
+    - uses: actions/checkout@v3
+    - name: Test Build
+      shell: bash
+      run: misc/macbuild.sh