test: make gno test without the specified target automatically infer "." to make it familiar for other Go contributors

[odeke-em]

Description

It is quite inconvenient to always have to specify gno test -v . when trying to run tests in a directory with the help message asking to specify a directory

Suggestion

Make it similar to the go test command that'll try to run within the current working directory if the package names aren't specified

This is an issue request @petar-dambovaliev @moul