test/test_collect: fix some timezone dependent tests

This test was using time.strftime which uses the current machine timezone,
but collection is done in UTC timezone, so the test was sometimes failing
because the strftime('%Y-%m-%d') was another day than the same time in
UTC time zone.

This test https://erp5js.nexedi.net/#/test_result_module/20210811-96D6B55F/8
for example had the problem. By using faketime and TZ=Europe/Paris
environment variable, the same problems reproduced locally 100% of times.

The fix is to always use datetime.utcnow().strftime('%Y-%m-%d'), to not
depend on the timezone.

slapos/tests/test_collect.py

@@ -42,7 +42,6 @@ import slapos.slap
 import psutil
 import sqlite3
 import subprocess
-from time import strftime
 from slapos.collect import entity, snapshot, db, reporter
 from slapos.cli.entry import SlapOSApp
 from six.moves.configparser import ConfigParser
@@ -247,7 +246,7 @@ class TestCollectDatabase(unittest.TestCase):
         self.assertEqual(len(database._getGarbageCollectionDateList(0)), 0)
 
         self.assertEqual(database._getGarbageCollectionDateList(1), 
-                           [strftime("%Y-%m-%d")])
+                           [datetime.utcnow().strftime("%Y-%m-%d")])
 
     def test_garbage(self):
 
@@ -378,9 +377,9 @@ class TestCollectReport(unittest.TestCase):
             self.assertEqual([i for i in csv.reader(csv_file)], csv_path_dict[f_path])
 
     def test_system_json_report(self):
-        database = self.getPopulatedDB(strftime("%Y-%m-%d"), amount=2)
+        database = self.getPopulatedDB(datetime.utcnow().strftime("%Y-%m-%d"), amount=2)
         reporter.SystemJSONReporterDumper(database).dump(self.instance_root)
-        date_to_test = strftime("%Y-%m-%d")
+        date_to_test = datetime.utcnow().strftime("%Y-%m-%d")
         json_path_dict = {
           '%s/system_memory_used.json' % self.instance_root: 
             '[  {    "entry": 0.09765625,     "time": "%s TIME"  }]' % date_to_test,
@@ -418,9 +417,9 @@ class TestCollectReport(unittest.TestCase):
 
 
     def test_system_csv_report(self):
-        database = self.getPopulatedDB(strftime("%Y-%m-%d"), amount=2)
+        database = self.getPopulatedDB(datetime.utcnow().strftime("%Y-%m-%d"), amount=2)
         reporter.SystemCSVReporterDumper(database).dump(self.instance_root)
-        date_for_test = strftime("%Y-%m-%d")
+        date_for_test = datetime.utcnow().strftime("%Y-%m-%d")
         csv_path_dict = {'%s/system_memory_used.csv' % self.instance_root:
                            [['time','entry'], ['%s TIME' % date_for_test,'0.09765625']],
                          '%s/system_cpu_percent.csv' % self.instance_root: