Add test for the integration with Solidus backend

[rainerdema]

Summary

Closes #150

The tests cover the following cases:	Description	Context	Expected Result	Pending
Navigates to the payments page	with successful payment operations	Payment is displayed	no
Displays log entries for a payment	with successful payment operations	Log entries are displayed	no
Captures an authorized payment successfully	with successful payment operations	Payment is successfully captured	no
Voids a payment from an order	with successful payment operations	Payment is successfully voided	no
Refunds a payment from an order	with successful payment operations	Payment is successfully refunded	no
Partially refunds a payment from an order	with successful payment operations	Payment is partially refunded	no
Cancels an order with captured payment	with successful payment operations	Payment is successfully voided	yes
Cancels an order with authorized payment	with successful payment operations	Payment is successfully voided	yes
Fails to capture a payment due to incomplete 3D Secure authentication	with failed payment operations	Payment capture fails and notification is displayed	no

Checklist

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[codecov[bot]]

Codecov Report

Merging #237 (df6af3a) into master (a54e2c8) will increase coverage by 1.42%. The diff coverage is 100.00%.

:exclamation: Current head df6af3a differs from pull request most recent head 152dd3f. Consider uploading reports for the commit 152dd3f to get more accurate results

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   98.35%   99.78%   +1.42%     
==========================================
  Files          23       24       +1     
  Lines         426      469      +43     
==========================================
+ Hits          419      468      +49     
+ Misses          7        1       -6     

Impacted Files	Coverage Δ
app/models/solidus_stripe/gateway.rb	100.00% <100.00%> (+3.38%)	:arrow_up:
lib/solidus_stripe/testing_support/factories.rb	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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