fix: ensure that error messages are always legible

[Kirpal]

Description

Fixes #63. The reason that error messages aren't legible in platform tests right now is that by default they try to use the 'monospace' font but we aren't loading a font for that by default, and Ahem is the fallback. I fixed it by changing the font family to Roboto for error widgets.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

Merging #78 (61779e7) into main (c5d14b8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #78   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          555       556    +1     
=========================================
+ Hits           555       556    +1     

Impacted Files	Coverage Δ
lib/src/golden_test.dart	100.00% <100.00%> (ø)

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