math: add a testcase for Mod and Remainder respectively

One might try to implement the Mod or Remainder function with the expression
x - TRUNC(x/y + 0.5)*y, but in fact this method is wrong, because the rounding
of (x/y + 0.5) to initialize the argument of TRUNC may lose too much precision.
However, the current test cases can not detect this error. This CL adds two
test cases to prevent people from continuing to do such attempts.

Change-Id: I6690f5cffb21bf8ae06a314b7a45cafff8bcee13
Reviewed-on: https://go-review.googlesource.com/84275Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/all_test.go

@@ -2435,6 +2435,10 @@ func TestMod(t *testing.T) {
 			t.Errorf("Mod(%g, %g) = %g, want %g", vffmodSC[i][0], vffmodSC[i][1], f, fmodSC[i])
 		}
 	}
+	// verify precision of result for extreme inputs
+	if f := Mod(5.9790119248836734e+200, 1.1258465975523544); 0.6447968302508578 != f {
+		t.Errorf("Remainder(5.9790119248836734e+200, 1.1258465975523544) = %g, want 0.6447968302508578", f)
+	}
 }
 
 func TestFrexp(t *testing.T) {
@@ -2776,6 +2780,10 @@ func TestRemainder(t *testing.T) {
 			t.Errorf("Remainder(%g, %g) = %g, want %g", vffmodSC[i][0], vffmodSC[i][1], f, fmodSC[i])
 		}
 	}
+	// verify precision of result for extreme inputs
+	if f := Remainder(5.9790119248836734e+200, 1.1258465975523544); -0.4810497673014966 != f {
+		t.Errorf("Remainder(5.9790119248836734e+200, 1.1258465975523544) = %g, want -0.4810497673014966", f)
+	}
 }
 
 func TestRound(t *testing.T) {