refactor: remove testify and transitive dependencies

[a-h]

Currently, go-serial has the following dependencies:

module go.bug.st/serial

go 1.17

require (
	github.com/creack/goselect v0.1.2
	github.com/stretchr/testify v1.8.4
	golang.org/x/sys v0.19.0
)

require (
	github.com/davecgh/go-spew v1.1.1 // indirect
	github.com/pmezard/go-difflib v1.0.0 // indirect
	gopkg.in/yaml.v3 v3.0.1 // indirect
)

If #149 is merged, goselect will disappear.

The project only uses a couple of functions from testify which are easy to refactor out to remove the other dependencies, leaving just golang.org/x/sys as a dependency, which would reduce the supply chain attack surface of the project.