when file not found in DEEPCELL_MESMER test, the test is not run, so the issue isn't reported

[RobJY]

PR checklist

• This comment contains a description of changes (with reason).
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).

I've changed to the tonsil file we usually use and new it's giving a new error, so looking into that as well.

So to be clear, there seem to be 2 issues.

1. If the file is not found the test is not run.
2. error generated from the file when found.

[nf-core-bot]

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[RobJY]

Interestingly, removing checkIfExists: true from the call to file() allows the test suite to catch the error.