Lint nf-test snapshots for the word "false"

[mahesh-panchal]

Description of feature

Sometimes we can only check if the contents of a file contains a specific string.
It can be quite easy to miss that the contains check when generating the snapshot was false

    "sarscov2 illumina paired-end [fastq_gz]": {
        "content": [
            [
                "versions.yml:md5,7b5a545483277cc0ff9189f8891e737f"
            ],
            [
                false
            ]
        ]

It would be good to fail linting in these cases.