Fix test failure on Windows caused by nil sys in mock FileInfo

[masih]

Fix test failure caused by panic when checking if a file is hidden in
Windows. The hidden check uses FileInfo.Sys(). In tests the object is
a mock and that call returns nil. Regardless of mocking, we should
check for nil since according to docs that function can return nil.

Run go mod tidy.

Relates to:

• sync: update CI config files #34

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