Migrate khmer/normalizebymedian to nf-test

[LouisLeNezet]

PR checklist

Closes #7603

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the module conventions in the contribution docs
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    • nf-core subworkflows test <SUBWORKFLOW> --profile singularity
    • nf-core subworkflows test <SUBWORKFLOW> --profile conda

[SPPearce]

I feel we should change the module here to use prefix.
This module apparently hasn't changed since the mass repo reorganisation almost 2.5 years ago.

[SPPearce]

I suspect the tests could just use the basic fastq files too, rather than requiring SEQTK_MERGEPE first (but provided they work I guess they are fine)

[SPPearce]

Ok, I think it wants interleaved fastq files for paired ends, where read1 and read2 come next to each other in one file.
To be honest, that is a much more sensible format than the two seperate files that are standard, but that's bioinformatics for you ;)