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

integration/util.py

@@ -0,0 +1,32 @@
+import time
+from os.path import exists
+
+
+def await_file_contents(path, contents, timeout=10):
+    start_time = time.time()
+    while time.time() - start_time < timeout:
+        print("  waiting for '{}'".format(path))
+        if exists(path):
+            with open(path, 'r') as f:
+                current = f.read()
+            if current == contents:
+                return True
+            print("  file contents still mismatched")
+            print("  wanted: {}".format(contents.replace('\n', ' ')))
+            print("     got: {}".format(current.replace('\n', ' ')))
+        time.sleep(1)
+    if exists(path):
+        raise Exception("Contents of '{}' mismatched after {}s".format(path, timeout))
+    raise Exception("Didn't find '{}' after {}s".format(path, timeout))
+
+
+def await_file_vanishes(path, timeout=10):
+    start_time = time.time()
+    while time.time() - start_time < timeout:
+        print("  waiting for '{}' to vanish".format(path))
+        if not exists(path):
+            return
+        time.sleep(1)
+    raise Exception("'{}' still exists after {}s".format(path, timeout))
+
+