Fix/failing times asserts in test discount

[korycins]

Quick fix for random failing tests from test_discount

[django-queries]

Here is the report for 298a8fb (mirumee/saleor @ fix/failing_times_asserts_in_test_discount)
Base comparison is 9dd1474.

No differences were found. (click me)

# api.benchmark checkout
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  add billing address to checkout     	         34	         34	             20
  add shipping to checkout            	          7	          7	              0
  checkout payment charge             	         14	         14	              0
  complete checkout                   	          6	          6	              0
  create checkout                     	         43	         43	             21

# api.benchmark homepage
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve main menu                  	          5	          5	              0
  retrieve product list               	          4	          4	              0
  retrieve secondary menu             	          5	          5	              0
  retrieve shop                       	          2	          2	              0

# api.benchmark product
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  product details                     	         13	         13	              3

# api.benchmark variant
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve variant list               	          9	          9	              2

[codecov]

Codecov Report

Merging #4401 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #4401      +/-   ##
=========================================
- Coverage   90.71%   90.7%   -0.02%     
=========================================
  Files         291     291              
  Lines       16679   16679              
  Branches     1691    1691              
=========================================
- Hits        15131   15129       -2     
- Misses       1053    1054       +1     
- Partials      495     496       +1

Impacted Files	Coverage Δ
saleor/payment/utils.py	91.07% <0%> (-0.94%)	⬇️

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[NyanKiyoshi]

c.f. #4397