Test2::Plugin::Cover will collect minimal file coverage info, and will do so
with minimal performance impact.

Every time a subroutine is called this tool will do its best to find the
filename the subroutine was defined in, and add it to a list. Also, anytime you
attempt to open a file with open() or sysopen() the file will be added to the
list. This list will be attached to a test2 event just before the test exits. In
most formaters the event will only show up as a comment on STDOUT # This test
covered N source files. . However tools such as Test2::Harness::UI can make full
use of the coverage information contained in the event.