Refactor "filer/tests/spec/fs.mkdir.spec.js" to follow ES6 syntax with "const/let" instead of "var"

[oleksii-polovyi]

fs.mkdir.spec.js file contains var declarations

In order to follow ES6 standards, the following changes are required:

• [x] Refactor the code to use let and const instead of var
• [x] Invoke a strict mode

[humphd]

Sounds good, go for it @ApolllonDev.

[oleksii-polovyi]

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