Improve GetChildren method in server tests

Use children method of Process class instead of iterating on all
processes.

ycmd/tests/server_test.py

@@ -178,11 +178,7 @@ def _DefaultSettings( self ):
 
 
   def _GetChildren( self ):
-    children = []
-    for proc in psutil.process_iter():
-      if proc.ppid() == self._popen_handle.pid:
-        children.append( proc )
-    return children
+    return psutil.Process( self._popen_handle.pid ).children()
 
 
   def _WaitUntilReady( self, timeout = 5 ):