all: do assorted cleanup, make tests pass

[josharian]

• add go.sum
• check error from os.RemoveAll in test
• make NewNotifyTest and friends handle cleanup
• use t.TempDir instead of ioutil.TempDir
• use os.SameFile to test for path equality
• be more careful about cleaning up test trees
• remove ioutil

[josharian]

This is work done in preparation for #212. I wanted to get all the tests passing before I started altering things, and got sidetracked by stranded test files and code modernization, and one thing led to another, and, well, there's a large PR. Sorry about that.

[rjeczalik]

@josharian Can you please squash all the commits into one?

[josharian]

Each commit is conceptually standalone, with an appropriate commit message...but if you would prefer, yes, I can squash them. I just wanted to confirm first. :)

[josharian]

OK, commits squashed down to one.