feat: add test class typed properties option

[paul-thebaud]

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Description

Resolves #20 by providing a new config option:

• testClassTypedProperties (defaults to false): Tells if the test class properties should be strictly typed.

Additional change: test class properties are now declared private. This is a breaking change (new option and multiple interface change), so it should be released on a new major release.

[sonarcloud[bot]]

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