test: add new pytest.ini and use --basetemp=/var/tmp by default

Our test images can easily be 10GB and if /tmp is mounted via a tmpfs
that can be too much for low memory systems. Hence switch to
/var/tmp by default.

Note that we do not really need 10GB as it's a sparse files but if
/tmp is too small we still get ENOSPC.

.github/workflows/tests.yml

@@ -112,4 +112,4 @@ jobs:
         AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
         AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
       run: |
-        pytest -rs -s -vv
+        pytest -rs -s -vv --basetemp="$TMPDIR"

pytest.ini

@@ -0,0 +1,4 @@
+[pytest]
+# do not use /tmp by default as it may be on a tempfs and our tests can
+# generate 10G images (that full of holes so not really 10G but still)
+addopts = --basetemp=/var/tmp