Add unit tests for fetch.py

R=ehmaldonado@chromium.org

Change-Id: I298d5f962e03520a93be1a7e32943934f1bde0c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2101809Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

fetch.py

@@ -55,14 +55,12 @@ class Checkout(object):
     self.root = root
 
   def exists(self):
+    """Check does this checkout already exist on desired location"""
     pass
 
   def init(self):
     pass
 
-  def sync(self):
-    pass
-
   def run(self, cmd, return_stdout=False, **kwargs):
     print('Running: %s' % (' '.join(pipes.quote(x) for x in cmd)))
     if self.options.dry_run: