Add unit tests for the bug in gh-46.

Author: WarrenWeckesser

tests/xsf_tests/README.md

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+These tests are independent of SciPy.

tests/xsf_tests/test_cyl_bessel_k.cpp

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+#include "../testing_utils.h"
+
+#include <complex>
+#include <tuple>
+#include <xsf/bessel.h>
+
+
+TEST_CASE("cyl_bessel_k gh-46", "[cyl_bessel_k][xsf_tests]") {
+    using test_case = std::tuple<double, std::complex<double>, std::complex<double>, double>;
+    auto [v, z, ref, rtol] = GENERATE(
+        test_case{0.0, std::complex{680.0, -1000.0},
+                  std::complex{1.901684871999608e-298, 1.713412341479591e-297}, 1e-13},
+        test_case{0.0, std::complex{680.0, -680.0}, 
+                  std::complex{-4.553730032944803e-298, 1.878727010109855e-297}, 1e-13},
+        test_case(0.0, std::complex{25.0, 100.0},
+                  std::complex{1.699267100365868e-12, -2.234890030902166e-13}, 5e-16)
+    );
+    const std::complex w = xsf::cyl_bessel_k(v, z);
+    const auto rel_error = xsf::extended_relative_error(w, ref);
+
+    CAPTURE(v, z, w, ref, rtol, rel_error);
+    REQUIRE(rel_error <= rtol);
+}