Refactor test cleanup in driver_test.go

[shogo82148]

Description

Please explain the changes you made here.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[coveralls]

coverage: 82.174% (-0.5%) from 82.681%
when pulling 5965ccb on parallelize-test
into fc589cb on master.