Collected fixes for incomplete beta in the asymptotic large a/b region

[jzmaddock]

Fixes #1198.

This change introduces some rather extreme new test cases, expect CI carnage!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.84%. Comparing base (e818106) to head (e8c86a8).
Report is 17 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1257   +/-   ##
========================================
  Coverage    93.83%   93.84%           
========================================
  Files          657      658    +1     
  Lines        55244    55295   +51     
========================================
+ Hits         51840    51891   +51     
  Misses        3404     3404           

Files with missing lines	Coverage Δ
...lude/boost/math/distributions/non_central_beta.hpp	92.09% <100.00%> (ø)
include/boost/math/special_functions/beta.hpp	100.00% <100.00%> (ø)
.../boost/math/special_functions/detail/bessel_ik.hpp	99.44% <100.00%> (ø)
test/test_ibeta.cpp	97.91% <100.00%> (+0.41%)	⬆️
test/test_ibeta.hpp	99.03% <100.00%> (+0.01%)	⬆️
test/test_nc_f.cpp	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[jzmaddock]

Just the sycl tests failing so merging...

[dschmitz89]

Thanks @jzmaddock ! Do you expect this to also close scipy/scipy#21725 ? Another case of large a and b.

[dschmitz89]

Thanks @jzmaddock ! Do you expect this to also close scipy/scipy#21725 ? Another case of large a and b.

Looks like it does not.