Make raw multisig tests legacy wallet only in rpc_rawtransaction.py

Summary:
The traditional multisig workflow doesn't work with descriptor wallets
so make these tests legacy wallet only.

This is a backport of core#18788 [9/15]
https://github.com/bitcoin/bitcoin/pull/18788/commits/47d3243160fdec7e464cfb8f869be7f5d4ee25fe

Notes for reviewers:

• the sanity checks introduced in D2546 belong to the indented code, so I had to move them up (move & indent only)
• the "RAW TX MULTISIG TESTS 2of2 tests" are indented only. The only changes are code formatting to keep lines below 80 characters.

Test Plan: test/functional/test_runner.py wallet_createwallet

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10671