test: skip if the namespace cannot be set up due to permissions

On some build systems, like Debian's, the tests do not have permissions to
create new namespaces, so skip gracefully in that case

Follow-up for 8e17f09

test/wrapper.py

@@ -17,17 +17,21 @@ def setup_test_namespace(data_dir):
     # Setup a new mount & user namespace, so we can use mount() unprivileged (see user_namespaces(7))
     euid = os.geteuid()
     egid = os.getegid()
-    os.unshare(os.CLONE_NEWNS|os.CLONE_NEWUSER)
-    # Map root to the original EUID and EGID, so we can actually call mount() inside our namespace
-    with open("/proc/self/uid_map", "w") as f:
-        f.write(f"0 {euid} 1")
-    with open("/proc/self/setgroups", "w") as f:
-        f.write("deny")
-    with open("/proc/self/gid_map", "w") as f:
-        f.write(f"0 {egid} 1")
-
-    # Overmount /etc with our own version
-    subprocess.check_call(["mount", "--bind", os.path.join(data_dir, "etc"), "/etc"])
+    try:
+        os.unshare(os.CLONE_NEWNS|os.CLONE_NEWUSER)
+        # Map root to the original EUID and EGID, so we can actually call mount() inside our namespace
+        with open("/proc/self/uid_map", "w") as f:
+            f.write(f"0 {euid} 1")
+        with open("/proc/self/setgroups", "w") as f:
+            f.write("deny")
+        with open("/proc/self/gid_map", "w") as f:
+            f.write(f"0 {egid} 1")
+
+        # Overmount /etc with our own version
+        subprocess.check_call(["mount", "--bind", os.path.join(data_dir, "etc"), "/etc"])
+    except PermissionError:
+        print("Lacking permissions to set up test harness, skipping")
+        sys.exit(77)
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser()