Merge bitcoin/bitcoin#25827: descriptor: check if `rawtr` has only one key.

416ceb8661117235c76f2985512473ebbc64956b descriptor: check if `rawtr` has only one key. (w0xlt)

Pull request description:

  If I understand `rawtr` descriptor correctly, it should only allow `rawtr(KEY)`, not `rawtr(KEY1, KEY2, ...)` or other concatenations.

  On master branch, `rawtr(KEY1, KEY2, ...)` will produce the `rawtr(KEY1)` descriptor ignoring the `KEY2, ...` with no error messages or warnings.

  For example, the code below will print `rawtr(tprv8ZgxMBicQKsPefef2Doobbq3xTCaVTHcDn6me82KSXY1vY9AJAWD5u7SDM4XGLfc4EoXRMFrJKpp6HNmQWA3FTMRQeEmMJYJ9RPqe9ne2hU/*)#lx9qryfh`
  for the supposedly invalid descriptor
  `rawtr(tprv8ZgxMBicQKsPefef2Doobbq3xTCaVTHcDn6me82KSXY1vY9AJAWD5u7SDM4XGLfc4EoXRMFrJKpp6HNmQWA3FTMRQeEmMJYJ9RPqe9ne2hU/*, tprv8ZgxMBicQKsPezQ2KGArMRovTEbCGxaLgBgaVcTvEx8mby8ogX2bgC4HBapH4yMwrz2FpoCuA17eocuUVMgEP6fnm83YpwSDTFrumw42bny/*)`
  ```python
          self.nodes[1].createwallet(wallet_name="rawtr_multi", descriptors=True, blank=True)
          rawtr_multi = self.nodes[1].get_wallet_rpc("rawtr_multi")
          rawtr_multi_desc = "rawtr(tprv8ZgxMBicQKsPefef2Doobbq3xTCaVTHcDn6me82KSXY1vY9AJAWD5u7SDM4XGLfc4EoXRMFrJKpp6HNmQWA3FTMRQeEmMJYJ9RPqe9ne2hU/*, tprv8ZgxMBicQKsPezQ2KGArMRovTEbCGxaLgBgaVcTvEx8mby8ogX2bgC4HBapH4yMwrz2FpoCuA17eocuUVMgEP6fnm83YpwSDTFrumw42bny/*)#uv78hkt0"
          result = rawtr_multi.importdescriptors([{"desc": rawtr_multi_desc, "active": True, "timestamp": "now"}])

          print(rawtr_multi.listdescriptors(True))
  ```

  This PR adds a check that prevents `rawtr` descriptors from being created if more than one key is entered, shows an error message, and adds a test for this case.

ACKs for top commit:
  achow101:
    ACK 416ceb8661117235c76f2985512473ebbc64956b
  sipa:
    ACK 416ceb8661117235c76f2985512473ebbc64956b

Tree-SHA512: a2009e91f1bca6ee79cc68f65811caa6a21fc8b80acd8dc58e283f424b41fe53b0db7ce3693b1c7e2184ff571e6d1fbb9f5ccde89b65d3026726f3393c492044

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