Add testing for File Provider Module

[claucambra]

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Feature description

As a major new sync engine for the desktop client, we should add testing to the file provider module to ensure file sync is happening as expected.

@tobiasKaminsky

[claucambra]

Estimation for this task: 3 weeks, mostly on work in the NextcloudFileProviderKit side (with some changes on the desktop client side/File Provider Module side to accomodate API changes)