Add testinfra check that the backup script works

Try creating a backup and verify it contains a few of the files we
expect it to.

molecule/testinfra/app/test_backup.py

@@ -0,0 +1,28 @@
+import pytest
+import testutils
+
+sdvars = testutils.securedrop_test_vars
+testinfra_hosts = [sdvars.app_hostname]
+
+
+@pytest.mark.skip_in_prod
+def test_backup(host):
+    """Create a backup and verify it contains expected files"""
+
+    with host.sudo():
+        result = host.run("securedrop-app-backup.py --dest /tmp")
+        assert result.rc == 0
+        tarball = result.stdout.strip()
+        # looks like a file path
+        assert tarball.endswith(".tar.gz")
+        assert host.file(f"/tmp/{tarball}").exists
+        # list files in the tarball
+        contains = host.run(f"tar -tzf /tmp/{tarball}")
+        assert contains.rc == 0
+        contains_list = contains.stdout.splitlines()
+        assert "/var/www/securedrop/config.py" in contains_list
+        assert "/etc/tor/torrc" in contains_list
+        assert "/var/lib/tor/services" in contains_list
+        # cleanup
+        cleanup = host.run(f"rm /tmp/{tarball}")
+        assert cleanup.rc == 0