Multiple test files

i had the following issue:
i have multiple files in `t/`. as `done_testing()` gets executed mulitple times. the current rex version throws errors like `not ok xx - done_testing() was already called a blah`. this pull request fixes this issue. i switched to `Test::Builder` as it is faster this way and it seems like `Rex::Test::Base` was not using any special `Test::More` functionality. the call to `Test::Builder::reset` fixes my original mentioned issue.