Trial functionality for running tests with pytest

* Change current dir to tmp dir on each test run
  * Log twisted with test.log

pytest.ini

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+
+[pytest]
+usefixtures = chdir setlog

scrapy/conftest.py

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+
+import pytest
+from twisted.python import log
+
+
+class LogObservers:
+    """Class for keeping track of log observers across test modules"""
+
+    def __init__(self):
+        self.observers = []
+
+    def add(self, logfile='test.log'):
+        fileobj = open(logfile, 'wb')
+        observer = log.FileLogObserver(fileobj)
+        log.startLoggingWithObserver(observer.emit, 0)
+        self.observers.append((fileobj, observer))
+
+    def remove(self):
+        fileobj, observer = self.observers.pop()
+        log.removeObserver(observer.emit)
+        fileobj.close()
+
+
+@pytest.fixture(scope='module')
+def log_observers():
+    return LogObservers()
+
+
+@pytest.fixture()
+def setlog(request, log_observers):
+    """Attach test.log file observer to twisted log, for trial compatibility"""
+    log_observers.add()
+    request.addfinalizer(log_observers.remove)
+
+
+@pytest.fixture()
+def chdir(tmpdir):
+    """Change to pytest-provided temporary directory"""
+    tmpdir.chdir()