Is it normal all testnet p2wsh cosigners have same master public key?
Not normal, each cosigner should have a unique master public key.

There is a reported bug in Electrum 3.3.6 that causes the issue on macOS.

Until a fix is pushed to production, it’s possible to get master public keys using the Electrum console (View/Show Console).

[k.xpub for k in wallet.get_keystores()]