Do fewer iterations in rsa_bench when running ctgrind tests.

[brian-pane]

This test takes a long time under Valgrind, so the number of iterations is reduced to one to speed it up.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.76%. Comparing base (c83b34d) to head (425b620).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #177   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files         184      184           
  Lines       50802    50802           
=======================================
+ Hits        50682    50684    +2     
+ Misses        120      118    -2     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 155 untouched benchmarks

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_{Comparing brian-pane:rsa-bench-perf (425b620) with main (c83b34d)}