ci: bump GitHub action versions in reusable_testing.yml

- actions/checkout: v2 -> v3
- actions/download-artifact: v2 -> v3

Author: Islam Elkanov

.github/workflows/reusable_testing.yml

@@ -14,12 +14,12 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - name: Clone the go-tarantool connector
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
         with:
           repository: ${{ github.repository_owner }}/go-tarantool
 
       - name: Download the tarantool build artifact
-        uses: actions/download-artifact@v2
+        uses: actions/download-artifact@v3
         with:
           name: ${{ inputs.artifact_name }}