CI: Rename the GitHub Actions workflow file tests.yaml

This is more logical and sets the ground for other GitHub Actions
workflows, e.g. automated distribution releases.

Author: adrienverge

.github/workflows/ci.yaml renamed to .github/workflows/tests.yaml

@@ -1,6 +1,6 @@
 ---
 
-name: CI
+name: Tests
 
 on:  # yamllint disable-line rule:truthy
   push:
@@ -33,7 +33,7 @@ jobs:
         run: make -C docs linkcheck
 
   test:
-    name: Tests
+    name: Unit tests
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false