GoAddTest, GoAddExpTest and GoAddAllTest affected.
Previously they weren't work with templates (go_nvim_test_template
variable) because file paths should be last argument to gotests.
Also removed different handling of stdout/stderr because seems that
gotests doesn't use stderr or produce any exit code except zero (that
probably OK, but not useful for go.nvim).
Also fixed README.md: description of GoAddTest, GoAddExpTest and
GoAddAllTest and setup example (go.nvim doesn't default test_template to
testify)