ci: test against multiple versions of @testing-library/dom

Author: G-Rath

.github/workflows/validate.yml

@@ -22,6 +22,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        testing-library: [8, 9]
         eslint: [6.8.0, 6, 7.0.0, 7, 8.0.0, 8]
         node: [12.22.0, 12, 14.17.0, 14, 16.0.0, 16]
     runs-on: ubuntu-latest
@@ -42,6 +43,9 @@ jobs:
       - name: Install ESLint v${{ matrix.eslint }}
         run: npm install --no-save --force eslint@${{ matrix.eslint }}
 
+      - name: Install @testing-library/dom v${{ matrix.testing-library }}
+        run: npm install --no-save --force @testing-library/dom@${{ matrix.testing-library }}
+
       - name: ▶️ Run validate script (without linting)
         if: ${{ matrix.eslint != 8 }}
         run: npm run validate -- build,test:coverage