run_test_file() duration longer than 1min are incorrectly displayed

`print.tinytest` expects duration attribute of output object to be in seconds. However, `run_test_file` produces a difftime object with `"auto"` units. 

Issue is here: https://github.com/markvanderloo/tinytest/blob/bcfec2d/pkg/R/tinytest.R#L698

This issue was fixed 4yrs ago for `run_test_dir`  b6d4589 , but remains for `run_test_file`. 
