unsafeslice: make TestStringAllocs more robust to inlining and DCE

Use the same technique as in the Benchmark functions to prevent the
compiler from eliminating the converted value, and thus potentially
also the conversion itself (especially in unsafe mode).

Tests still pass alll the way back to go1.9.7.

unsafeslice_test.go

@@ -162,19 +162,27 @@ func ExampleAsString() {
 func TestStringAllocs(t *testing.T) {
 	t.Run("OfString", func(t *testing.T) {
 		s := "Hello, world!"
+
+		var b []byte
 		avg := testing.AllocsPerRun(1000, func() {
-			_ = unsafeslice.OfString(s)
+			b = unsafeslice.OfString(s)
 		})
+		runtime.KeepAlive(b)
+
 		if avg > 0.01+maxStringAllocs {
 			t.Errorf("unsafeslice.OfString made %v allocations; want %d", avg, maxStringAllocs)
 		}
 	})
 
 	t.Run("AsString", func(t *testing.T) {
 		b := []byte("Hello, world!")
+
+		var s string
 		avg := testing.AllocsPerRun(1000, func() {
-			_ = unsafeslice.AsString(b)
+			s = unsafeslice.AsString(b)
 		})
+		runtime.KeepAlive(s)
+
 		if avg > 0.01+maxStringAllocs {
 			t.Errorf("unsafeslice.OfString made %v allocations; want %d", avg, maxStringAllocs)
 		}