Test user provided execute implementations

[crhntr]

Users of muxt can bring their own execute function.
We should have some expectations for what these functions do.

For example they should not call WriteHeader if they are passed
the value false for parameter writeHeader.

Given I import "GitHub.com/crhntr/muxt/muxttest" into my test package
And I call `muxttest.Execute(t, execute)`
And `execute` matches expectations for a user provided execute func
When I run `go test`
Then the tests pass

Maybe this is doing too much. Unfortunately this design would require
the library to import muxt. I don't like that. So maybe we should generate tests instead.