Add xtensor back for integration testing

Author: chrisrichardson

.github/workflows/dolfin-tests.yml

@@ -76,6 +76,7 @@ jobs:
           ref: ${{ github.event.inputs.dolfinx_branch }}
       - name: Install DOLFINx
         run: |
+          apt-get install libxtensor-dev
           cmake -G Ninja -DCMAKE_BUILD_TYPE=Developer -B build -S dolfinx/cpp/
           cmake --build build
           cmake --install build