fix(test-specs): parenthesize walrus when counting failing txs

[cuiweixie]

Description

Fix operator precedence around the walrus assignment when counting failing transactions:

# Before (wrong): assigns bool because `>` binds tighter than `:=`
if failing_tx_count := len([...]) > 0:

# After (correct): assigns the count, then compares
if (failing_tx_count := len([...])) > 0:

Previously failing_tx_count was always True/False, so failing_tx_count > 1 never detected multiple failing transactions and the correctness check was effectively dead.

Related Issues or PRs

N/A.

Checklist

• Ran fast static checks to avoid CI fails, see Code Standards & Verifying Changes: just static
• PR title has the form <type>(<area>): <title>, where <type> and <area> come from an appropriate C-<type>, respectively A-<area>, label. The title should match the target squash commit message.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.39%. Comparing base (2119b38) to head (d11e691).

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #3190   +/-   ##
================================================
  Coverage            93.39%   93.39%           
================================================
  Files                  624      624           
  Lines                37014    37014           
  Branches              3386     3386           
================================================
  Hits                 34570    34570           
  Misses                1671     1671           
  Partials               773      773           

Flag	Coverage Δ
unittests	93.39% <ø> (ø)

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