This re-factors the magic-folder tests to abstract
the whole "do a file operation" so we can properly
send fake (or wait for real) inotify events to the
uploader/downloader. This speeds up the tests quite
a bit and makes test_alice_bob reasonable again (at
about 1.5s instead of over 30s).
Most of the CLI tests don't care about the actual shares. Configuring
the test client to use k=N=1 reduces the runtime from 180s to 90s on my
laptop.
A few tests *do* care, like test_check (which delete some shares, then
assert that 'tahoe check' shows the damage). These still use k=3/N=10.
