ci: check spelling via codespell

Use the github action[1] provided by the codespell project to
check the spelling on every PR.

[1] https://github.com/codespell-project/actions-codespell

.github/workflows/tests.yml

@@ -94,3 +94,13 @@ jobs:
         run: |
           make --always-make test-data
           git diff --exit-code -- ./test/data
+
+  codespell:
+    name: "Spell check"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: codespell-project/actions-codespell@master
+        with:
+          ignore_words_list: msdos
+          skip: ./.git,coverity,rpmbuild,samples