add missing checkout action

.github/workflows/tests.yml

@@ -8,12 +8,14 @@ jobs:
   test-normal-compile:
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v3
       - name: Run tests with standard compilation flags
         working-directory: tests/
         run: ./normal_compile.sh
   test-optimized-compile:
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v3
       - name: Run tests with optimized compilation flags
         working-directory: tests/
         run: ./optimized_compile.sh