Fix flaky DER signature weight test

[t-bast]

This test was flaky because for the funding output, we provide two signatures, which means that we need twice the slack for the variable DER encoding length.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.64%. Comparing base (129df36) to head (955c34f).
⚠️ Report is 16 commits behind head on master.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3299      +/-   ##
==========================================
- Coverage   88.67%   88.64%   -0.04%     
==========================================
  Files         221      222       +1     
  Lines       21453    21571     +118     
  Branches      832      868      +36     
==========================================
+ Hits        19023    19121      +98     
- Misses       2430     2450      +20     

see 9 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.