math: add more tests for special cases of Bessel functions Y0, Y1, Yn

Test finite negative x with Y0(-1), Y1(-1), Yn(2,-1), Yn(-3,-1).

Also test the special case Yn(0,0).

Fixes #19130.

Change-Id: I95f05a72e1c455ed8ddf202c56f4266f03f370fd
Reviewed-on: https://go-review.googlesource.com/37310Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/all_test.go

@@ -1742,30 +1742,35 @@ var vfy0SC = []float64{
 	0,
 	Inf(1),
 	NaN(),
+	-1,
 }
 var y0SC = []float64{
 	NaN(),
 	Inf(-1),
 	0,
 	NaN(),
+	NaN(),
 }
 var y1SC = []float64{
 	NaN(),
 	Inf(-1),
 	0,
 	NaN(),
+	NaN(),
 }
 var y2SC = []float64{
 	NaN(),
 	Inf(-1),
 	0,
 	NaN(),
+	NaN(),
 }
 var yM3SC = []float64{
 	NaN(),
 	Inf(1),
 	0,
 	NaN(),
+	NaN(),
 }
 
 // arguments and expected results for boundary cases
@@ -2716,6 +2721,9 @@ func TestYn(t *testing.T) {
 			t.Errorf("Yn(-3, %g) = %g, want %g", vfy0SC[i], f, yM3SC[i])
 		}
 	}
+	if f := Yn(0, 0); !alike(Inf(-1), f) {
+		t.Errorf("Yn(0, 0) = %g, want %g", f, Inf(-1))
+	}
 }
 
 // Check that math functions of high angle values