[FIX] test_owfile: Fix unreliable test_warning_from_another_thread

[ales-erjavec]

Issue

Fix intermittent test failure:

FAIL: test_warning_from_another_thread (Orange.widgets.data.tests.test_owfile.TestOWFile)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/runner/hostedtoolcache/Python/3.10.5/x64/lib/python3.10/unittest/mock.py", line 1369, in patched
    return func(*newargs, **newkeywargs)
  File "/Users/runner/work/orange3/orange3/.tox/orange-released/lib/python3.10/site-packages/Orange/widgets/data/tests/test_owfile.py", line 693, in test_warning_from_another_thread
    with self.assertWarns(UserWarning):
AssertionError: UserWarning not triggered

Description of changes

Replace use of time.sleep to sequence threaded code.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6085 (8c1fdb3) into master (04c0e73) will increase coverage by 0.02%.
The diff coverage is 96.61%.

@@            Coverage Diff             @@
##           master    #6085      +/-   ##
==========================================
+ Coverage   86.59%   86.62%   +0.02%     
==========================================
  Files         315      315              
  Lines       67317    67479     +162     
==========================================
+ Hits        58295    58454     +159     
- Misses       9022     9025       +3