tests: Add test for math special functions.

tests/float/math_fun_special.py

+# test the special functions imported from math
+
+try:
+    from math import *
+except ImportError:
+    print("SKIP")
+    import sys
+    sys.exit()
+
+test_values = [-8., -2.5, -1, -0.5, 0.0, 0.5, 2.5, 8.,]
+pos_test_values = [0.001, 0.1, 0.5, 1.0, 1.5, 10.,]
+
+functions = [
+    ('erf', erf, test_values),
+    ('erfc', erfc, test_values),
+    ('gamma', gamma, pos_test_values),
+    ('lgamma', lgamma, pos_test_values + [50., 100.,]),
+]
+
+for function_name, function, test_vals in functions:
+    print(function_name)
+    for value in test_vals:
+        print("{:.5g}".format(function(value)))