Fix the txmaker test util

[bigspider]

Bug in a utility function only used in tests; it would scramble the order of the keys, producing incorrect PSBTs. Not caught earlier as it only shows up on wallet policies with multiple key placeholders, and all the tests using the txmaker use policies with a single key.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.89%. Comparing base (e589d75) to head (e9d3a4b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #260 +/- ## ======================================== Coverage 84.89% 84.89% ======================================== Files 17 17 Lines 2205 2205 ======================================== Hits 1872 1872 Misses 333 333 ``` | [Flag](https://app.codecov.io/gh/LedgerHQ/app-bitcoin-new/pull/260/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/LedgerHQ/app-bitcoin-new/pull/260/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ) | `84.89% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.