Increase test coverage, fix NaN

[AshtonSBradley]

This PR increases test coverage to address #30. Coverage is up ~3% for the package and special cases are addressed more thoroughly. @dlfivefifty @MikaelSlevinsky I hope this helps package robustness.

This PR should also close #73 and #79. Both were caused by finite terminating hypergeometric series being routed through generic evaluation paths that could hit 0/0-style intermediate states and return NaN; this branch now detects those terminating cases and evaluates the finite sum directly.

It also adds regression coverage for the _₂F₁ special-case behaviour tied to #86 so should help resolve it.

Coverage-wise, this PR also helps address #30 by adding direct tests for internal helper functions, explicit _₂F₁ special-case branches, and issue-driven regression blocks. Improved coverage also directly helps #61, #63, #40

It reorganizes the suite so coverage is easier to reason about and maintain. The old monolithic test/runtests.jl is now split into feature-focused files for special-function helpers, broad algorithm sweeps, confluent-function coverage, and regression/boundary tests. That makes it much easier to see what behavior each block is protecting and to add issue-driven tests without burying them in a giant file.

It adds targeted high-signal tests around behavior that was previously thinly covered. In particular:

• exact identity tests for short-circuit paths like M(a,a,z) = exp(z), pFq((3,),(),z) = (1-z)^(-3), and several _₂F₁ dispatch identities
• singular and near-singular parameter checks, including DomainError behavior, Inf/NaN cases at argument unity, and comparisons against BigFloat near problematic parameter values
• branch-cut consistency tests that verify conjugation symmetry for real-parameter evaluations of ₁F₀, M, and _₂F₁ when approaching the real axis from above and below
• dedicated confluent coverage for U, plus new regression-oriented checks around nearby sensitive parameter regimes.

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.88%. Comparing base (1bea052) to head (2dd7b56).

Files with missing lines	Patch %	Lines
src/specialfunctions.jl	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   84.89%   87.88%   +2.98%     
==========================================
  Files           7        7              
  Lines        1470     1502      +32     
==========================================
+ Hits         1248     1320      +72     
+ Misses        222      182      -40     

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