Enable mocking entire api client

[julianlocke]

see dropbox/SwiftyDropbox#394

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 (eb375a6) 50.28%.
Report is 9 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     #316      +/-   ##
=======================================================
- 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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