make fxs tokens usable for tests

[sparrowDom]

This fixes a test.

FXS when performing a transfer also creates voting snapshots. This uses FXS owner tokens instead of setting ERC20 balance via snapshots.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (826735c) 74.09% compared to head (5d3cfe3) 73.89%.

Additional details and impacted files

@@                      Coverage Diff                      @@
##           nicka/convex-frax-locking    #1981      +/-   ##
=============================================================
- Coverage                      74.09%   73.89%   -0.20%     
=============================================================
  Files                             61       61              
  Lines                           3015     3015              
  Branches                         781      781              
=============================================================
- Hits                            2234     2228       -6     
- Misses                           778      784       +6     
  Partials                           3        3              

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