Move check_magicfolder_smoke.py to proper integration tests

This introduces a py.test-based integration suite (currently just
containing magic-folder end-to-end tests). Also adds a tox environment
("integration") to run them.

The test setup is:

 - a "flogtool gather" instance
 - an Introducer
 - five Storage nodes
 - Alice and Bob client nodes
 - Alice and Bob have paired magic-folders

src/allmydata/_auto_deps.py

@@ -89,6 +89,9 @@ install_requires = [
     #   <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2474>.
     "pyOpenSSL >= 0.14",
     "PyYAML >= 3.11",
+
+    # in Python 3.3 stdlib
+    "shutilwhich >= 1.1.0",
 ]
 
 # Includes some indirect dependencies, but does not include allmydata.