Add an IsAttrib method on the FileEvent struct

This IsAttrib function can be used to distinguish events that only
concern a file's metadata (eg. atime, mtime etc.).
tests: Rename attribReceived to modifyReceived in TestFsnotifyAttrib

This counter is counting "IsModify" events.
tests: Allow resetting the value of a counter

This is useful when you want to check the event count delta after doing
some operations.
Add tests for the IsAttrib method