[tests]: fix drops_group_test failure on second run (sonic-net#1023)

a few tests in drops_group_test fails on second run.
The reason is that /tmp/dropstat is not cleaned, the first
run leave some state in the folder which cause the subsequent
run to fail.

The fix is the always clean up the folder.

Signed-off-by: Guohan Lu <lguohan@gmail.com>

tests/drops_group_test.py

@@ -1,5 +1,7 @@
-import sys
 import os
+import sys
+
+import shutil
 from click.testing import CliRunner
 
 test_path = os.path.dirname(os.path.abspath(__file__))
@@ -8,7 +10,6 @@
 sys.path.insert(0, test_path)
 sys.path.insert(0, modules_path)
 
-import mock_tables.dbconnector
 import show.main as show
 import clear.main as clear
 
@@ -75,10 +76,14 @@
 sonic_drops_test               0
 """
 
+dropstat_path = "/tmp/dropstat"
+
 class TestDropCounters(object):
     @classmethod
     def setup_class(cls):
         print("SETUP")
+        if os.path.exists(dropstat_path):
+            shutil.rmtree(dropstat_path)
         os.environ["PATH"] += os.pathsep + scripts_path
         os.environ["UTILITIES_UNIT_TESTING"] = "1"