Initial Test for `PayIDClient`

[keefertaylor]

High Level Overview of Change

Add testing infrastructure for PayID.

Context of Change

Previous versions of Swagger were untestable. #227 migrated to a new generator which has testable code.

This PR does a few things:

• Injects SessionManager into PayIDClient in the initializer
• Creates a FakeURLProtocol so that we can fake networking
• Hooks up a basic test

The faking code is heavily adapted from this blog post: https://www.google.com/search?q=alamofire+fake&oq=alamofire+fake&aqs=chrome..69i57.2366j0j1&sourceid=chrome&ie=UTF-8.

Since it is mostly boiler plate and it's only test code, I didn't do much in the way of documentation or stylistic updating.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Refactor (non-breaking change that only restructures code)
• [x] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation Updates
• [ ] Release

Before / After

Better test coverage.

Test Plan

New test passes on CI.

Future Tasks

We should continue building tests for PayID. They are omitted here for brevity of this PR. Once the testing infrastructure exists it is trivial to write additional test cases.

[codecov[bot]]

Codecov Report

Merging #238 into master will increase coverage by 1.22%. The diff coverage is 92.75%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   88.03%   89.26%   +1.22%     
==========================================
  Files          42       42              
  Lines         861      941      +80     
==========================================
+ Hits          758      840      +82     
+ Misses        103      101       -2     

Impacted Files	Coverage Δ
XpringKit/PayID/XRPPayIDClient.swift	85.00% <82.60%> (+3.18%)	:arrow_up:
XpringKit/PayID/PayIDClient.swift	91.95% <91.30%> (+10.70%)	:arrow_up:
XpringKit/XRP/DefaultXRPClient.swift	98.60% <100.00%> (+1.43%)	:arrow_up:
...XRP/Model/XRPPayment+Org_Xrpl_Rpc_V1_Payment.swift	100.00% <100.00%> (ø)
...l/XRPTransaction+Org_Xrpl_Rpc_V1_Transaction.swift	96.00% <100.00%> (+0.44%)	:arrow_up:
XpringKit/XRP/ReliableSubmissionXRPClient.swift	94.23% <100.00%> (+0.11%)	:arrow_up:
XpringKit/XRP/XRPClient.swift	100.00% <100.00%> (ø)
XpringKit/Xpring/XpringClient.swift	100.00% <100.00%> (+5.55%)	:arrow_up:

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