Add requires to nf-test.config in order to specify minimal nf-test version

[lukfor]

This PR introduces a new keyword requires in nf-test.config to to specify the minimum required version of nf-test. This fixes #160.

For instance, to ensure the use of at least nf-test version 0.9.0, define it as follows:

config {
    requires (
        "nf-test": "0.9.0"
    )
}

Open points:

• Write testcases