flag in test causes other tests to fail to run

I have a test file that I want to only run when a bit flag is set. I followed the simple example from golang's testing docs:

package mypkg

var myFlagSet = flag.Bool("myflag", false, "")
func TestMain(m *testing.M) {
    flag.Parse()
    if *myFlagSet {
        os.Exit(m.Run())
    }
}

if I run go test ./mypkg -myflag it runs as expected. However, when I run go test ./... -myflag, all other package tests fail:

flag provided but not defined: -myflag

I want to be able to run all tests and not have to worry about parsing this flag in every test file. Is there a way to do this that I'm missing?