backups: schedule: tests: Fix failures due to long test run

Closes: #2023.

When importing the test module, datetime.now() is executed and value is kept. If
a test suite runs for a long time, the time deltas are being calculated much
later when the test case runs. This creates an difference in expected different
between the two values.

Fix this by completely removing all uses of time relative to current date time.
Only use absolute date time values. This should not reduce the effectiveness of
the test cases.

Tests performed:

- Rerun unit tests for backups module.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/backups/tests/test_schedule.py

@@ -83,9 +83,7 @@ def _get_archives_from_test_data(data):
     archives = []
     for index, item in enumerate(data):
         archive_time = item['time']
-        if isinstance(archive_time, timedelta):
-            archive_time = datetime.now() + archive_time
-        elif isinstance(archive_time, str):
+        if isinstance(archive_time, str):
             archive_time = datetime.strptime(archive_time,
                                              '%Y-%m-%d %H:%M:%S+0000')
         archive = {
@@ -114,7 +112,7 @@ cases = [
     [
         [False, 10, 10, 10, 0],
         [],
-        datetime.now(),
+        datetime(2021, 1, 1),
         [],
         [],
     ],
@@ -122,7 +120,7 @@ cases = [
     [
         [True, 10, 10, 10, 0],
         [],
-        datetime.now(),
+        datetime(2021, 1, 1),
         ['daily', 'weekly', 'monthly'],
         [],
     ],
@@ -131,9 +129,9 @@ cases = [
         [True, 10, 10, 10, 0],
         [{
             'periods': ['daily'],
-            'time': timedelta(seconds=-600)
+            'time': datetime(2021, 1, 1) - timedelta(seconds=600)
         }],
-        datetime.now(),
+        datetime(2021, 1, 1),
         [],
         [],
     ],
@@ -142,9 +140,9 @@ cases = [
         [True, 10, 10, 10, 0],
         [{
             'periods': ['weekly'],
-            'time': timedelta(seconds=-600)
+            'time': datetime(2021, 1, 1) - timedelta(seconds=600)
         }],
-        datetime.now(),
+        datetime(2021, 1, 1),
         [],
         [],
     ],
@@ -153,9 +151,9 @@ cases = [
         [True, 10, 10, 10, 0],
         [{
             'periods': ['monthly'],
-            'time': timedelta(seconds=-600)
+            'time': datetime(2021, 1, 1) - timedelta(seconds=600)
         }],
-        datetime.now(),
+        datetime(2021, 1, 1),
         [],
         [],
     ],