feat: add flag to execute filetests with prefix

[deelawn]

This simply allows a prefix to be set when running filetests so that only a subset of tests are executed. This is especially useful when a PR requires many new filetests -- it allows execution of all new tests without having to wait for ALL tests to run.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.85%. Comparing base (5450f64) to head (a179338).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2837   +/-   ##
=======================================
  Coverage   60.85%   60.85%           
=======================================
  Files         563      563           
  Lines       75193    75193           
=======================================
+ Hits        45757    45759    +2     
+ Misses      26069    26066    -3     
- Partials     3367     3368    +1     

Flag	Coverage Δ
contribs/gnodev	60.65% <ø> (ø)
contribs/gnofaucet	15.31% <ø> (+0.85%)	⬆️
gno.land	67.17% <ø> (ø)
gnovm	65.63% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (+0.35%)	⬆️
tm2	61.97% <ø> (-0.10%)	⬇️

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[ltzmaxwell]

can we also have this for txtar?