Try and make some free space to allow SyTest to run in GHA

Author: reivilibre

.github/workflows/tests.yml

@@ -219,6 +219,8 @@ jobs:
             redis: redis
 
     steps:
+      - name: Make free disk space # https://github.com/actions/virtual-environments/issues/2840#issuecomment-858563680
+        run: sudo rm -rf /usr/share/dotnet
       - uses: actions/checkout@v2
       - name: Prepare test blacklist
         run: cat sytest-blacklist .ci/worker-blacklist > synapse-blacklist-with-workers