test: Add test for export-roles --indent's argument “duck casting” to int

[EBoisseauSierra]

Description

#1724 introduced the --indent argument to fab export-roles. This (optional) argument accepts int and str args — and notably negative integers.

However, fab export-roles --indent=-1 would naturally return -1… as a str, and not a negative int. That's why we “duck cast” the arg to an integer… if possible.

This piece of logic remained however untested. So for the sake of the exercise, this PR:

• extracts the duck casting logic into an ad hoc method,
• create tests for this method.

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Merging #1811 (5e1e134) into master (8a18281) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1811   +/-   ##
=======================================
  Coverage   77.11%   77.12%           
=======================================
  Files          56       56           
  Lines        8229     8232    +3     
=======================================
+ Hits         6346     6349    +3     
  Misses       1883     1883           

Flag	Coverage Δ
python	77.12% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flask_appbuilder/cli.py	58.92% <100.00%> (+0.55%)	⬆️

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[dpgaspar]

Awesome! thank you