POSIX: add simple bessel function tests

ext/POSIX/t/math.t

@@ -296,4 +296,30 @@ SKIP: {
     ok(signbit(copysign(NAN, -1.0)), "signbit(copysign(NAN, -1.0)))");
 }
 
+SKIP: {
+    # win32 msvcrt and ucrt both have these as _j0() etc, but might
+    # not make them visible in the headers
+    $Config{d_j0} || $^O eq "MSWin32"
+      or skip "No bessel functions", 1;
+    # just in case j0 etc ends up being called without a prototype
+    is_float(j0(0.5), 0.938469807240813, "j0");
+    is_float(j1(0.5), 0.242268457674874, "j1");
+    is_float(jn(1, 0.5), j1(0.5), "jn");
+    is_float(y0(0.5), -0.444518733506707, "y0");
+    is_float(y1(0.5), -1.47147239267024, "y1");
+    is_float(yn(1, 0.5), y1(0.5), "yn");
+}
+
+sub is_float {
+    my ($left, $right, $note) = @_;
+
+    my $ok = ok(abs($left - $right) < 0.00001, $note);
+    unless ($ok) {
+        diag <<EOS;
+ Expected: $left
+      Got: $right
+EOS
+    }
+}
+
 done_testing();