test: add custom TestCase class and use in stock module

[ankush]

Problem: Tests leave too much state in db which makes next tests flaky and dependent on state.

Solution:

• Create a custom test case class and stop committing in inheriting classes.
• Expand the reach of this class slowly till all major tests are covered. Starting with stock related tests for now because it's a pressing requirement to merge 2 other PRs.

[codecov]

Codecov Report

Merging #27496 (22c658b) into develop (c6a56a8) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop   #27496      +/-   ##
===========================================
+ Coverage    54.39%   54.44%   +0.04%     
===========================================
  Files         1254     1254              
  Lines        67725    67735      +10     
===========================================
+ Hits         36839    36877      +38     
+ Misses       30886    30858      -28     

Impacted Files	Coverage Δ
erpnext/tests/utils.py	98.11% <100.00%> (+0.43%)	⬆️
erpnext/stock/report/stock_ledger/stock_ledger.py	77.50% <0.00%> (-3.34%)	⬇️
...xt/stock/report/stock_analytics/stock_analytics.py	90.09% <0.00%> (-2.98%)	⬇️
erpnext/stock/report/stock_ageing/stock_ageing.py	88.48% <0.00%> (-2.88%)	⬇️
...value/warehouse_wise_item_balance_age_and_value.py	92.59% <0.00%> (-2.47%)	⬇️
erpnext/accounts/deferred_revenue.py	75.22% <0.00%> (-0.89%)	⬇️
erpnext/stock/stock_ledger.py	86.66% <0.00%> (-0.73%)	⬇️
...rpnext/stock/report/stock_balance/stock_balance.py	78.82% <0.00%> (-0.59%)	⬇️
erpnext/selling/doctype/sales_order/sales_order.py	76.39% <0.00%> (-0.38%)	⬇️
...xt/accounts/doctype/sales_invoice/sales_invoice.py	76.75% <0.00%> (+0.08%)	⬆️
... and 14 more

[ankush]

will do it the "proper way" and reopen later. 😄