[QA] Renaming rawtx into rawTx

Summary:
Follow up from #10069

fundrawtransaction.py is using two variables: rawtx and rawTx, which can be easily confused (see #10069). This PR rename everything in rawtx.

Inverted backport of Core PR10083 (rawtx -> rawTx instead of rawTx -> rawtx)
Depends on D2522

Test Plan: test_runner.py rpc_fundrawtransaction

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2523