Add tests

[eviweb]

Hi, this refers to this message: https://github.com/joh/when-changed/pull/77#discussion_r269892135

As some behaviors may differ from a platform to another, it would be great to add a bunch of tests to guarantee that the when-changed behaviors are the expected ones.

The expected behaviors are the following:

When watching directory:

• Existing file is modified
• File is created in the directory
• File is moved into the directory
• File is moved out of the directory
• File is deleted from the directory

When watching a file:

• File is modified
• The file is replaced with another file (overwritten)
• File is renamed
• File is deleted

[eviweb]

Motivated by PR #77