Testing package in Go

I am trying to grasp my skill on testing in go and not really understand why my code isn't tested.

so I have one package, two go files and two test files (a.go, a_test.go, b.go, and b_test.go).

but if I run go test -v ., as result are tests in file a_test are run. Why is that? What can I do to achieve the right practices for testing?

P.S. this is package in one directory, and it's not insalled.