[1.x] Fix undefined relations to provider & merchant while using config driver.

[r-kujawa]

What does this PR do? 🤖

• Allows for gateway requests to be auto-configured within the Wallet::class & PaymentMethod::class models using any driver.
• Improves code coverage by adding and reorganizing tests.

Any background context you would like to provide? 🚧

This bug was introduced when we added support from only database drivers to custom drivers, including the config driver.

Does this relate to any issue? 🔗

Closes #34

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +3.33 🎉

Comparison is base (8856202) 56.13% compared to head (3fea79f) 59.46%.

Additional details and impacted files

@@             Coverage Diff              @@
##                1.x      #37      +/-   ##
============================================
+ Coverage     56.13%   59.46%   +3.33%     
- Complexity      224      226       +2     
============================================
  Files            42       42              
  Lines           709      713       +4     
============================================
+ Hits            398      424      +26     
+ Misses          311      289      -22     

Impacted Files	Coverage Δ
src/Models/PaymentMethod.php	50.00% <100.00%> (+33.33%)	⬆️
src/Models/Traits/ConfiguresPaymentGateway.php	100.00% <100.00%> (+100.00%)	⬆️
src/database/factories/PaymentProviderFactory.php	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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