100% Test coverage all (?) distributions

[JacobHass8]

Added tests for the following distributions to get 100% test coverage.

• arcsine
• bernoulli (97%)
• beta (99%)

Here is the code coverage report for my information.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.42%. Comparing base (592063d) to head (264487e).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1399      +/-   ##
===========================================
+ Coverage    95.35%   95.42%   +0.06%     
===========================================
  Files          825      825              
  Lines        68310    68386      +76     
===========================================
+ Hits         65139    65259     +120     
+ Misses        3171     3127      -44     

Files with missing lines	Coverage Δ
test/test_arcsine.cpp	99.24% <100.00%> (+0.06%)	⬆️
test/test_bernoulli.cpp	100.00% <100.00%> (ø)
test/test_beta_dist.cpp	98.95% <100.00%> (+0.24%)	⬆️

... and 4 files with indirect coverage changes

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