unix: test (*FdSet).Clear in TestFdSet

Follow-up for CL 205397 so all *FdSet functionality is tested.

Change-Id: Ieb5dce6ad020ce1252cae7f3fe96ba6bf0069f5e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/215241
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/fdset_test.go

@@ -50,4 +50,14 @@ func TestFdSet(t *testing.T) {
 			}
 		}
 	}
+
+	for fd := 1; fd < unix.FD_SETSIZE; fd += 2 {
+		fdSet.Clear(fd)
+	}
+
+	for fd := 0; fd < unix.FD_SETSIZE; fd++ {
+		if fdSet.IsSet(fd) {
+			t.Fatalf("Clear(%d) did not clear fd", fd)
+		}
+	}
 }