Swift test mocking enchancements

[julianlocke]

See dropbox/SwiftyDropbox#392 for generated code output.

Make route api objects conform to JSONRepresentable, and route namespace objects conform to DropboxTransportClientOwning.

Avoid stack overflows in description vars that fallback on themselves when serialization errors.

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (94fa542) 50.30% compared to head (f02b6de) 50.28%.
Report is 8 commits behind head on swifty-dropbox-beta.

Files	Patch %	Lines
stone/cli.py	0.00%	16 Missing ⚠️
stone/backends/swift_client.py	0.00%	1 Missing ⚠️
stone/frontend/ir_generator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           swifty-dropbox-beta     #315      +/-   ##
=======================================================
- Coverage                50.30%   50.28%   -0.03%     
=======================================================
  Files                       37       37              
  Lines                     8740     8744       +4     
  Branches                  1881     1881              
=======================================================
  Hits                      4397     4397              
- Misses                    4026     4030       +4     
  Partials                   317      317              

Flag	Coverage Δ
unit	50.28% <5.26%> (-0.03%)	⬇️

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[jaquish]

LGTM