Add nf-test for local modules - or move to modules

[maxulysse]

Description of feature

(proposed destinations added by @pinin4fjords , and subject to discussion)
(discussion done with @maxulysse)

• Migrate bedtools/genome_cov to nf-core/modules #1161 bedtools_genomecov/main.nf - (-> modules)
• Migrate CAT_ADDITIONAL_FASTA module to nf-core/modules #1162 cat_additional_fasta/main.nf - (stay local, or in modules under 'custom')
• Add nf-test to DESEQ2_QC module #1163 deseq2_qc/main.nf - (probably tricky to go to modules. It's not actually much to do with DESeq2, just plots and such.)
• Migrate DUPRADAR to nf-core/modules #1164 dupradar/main.nf - (-> modules, probably using R template pattern)
• Migrate gtf2bed to nf-core/modules #1165 gtf2bed/main.nf - (-> modules under custom/) - this one was borrowed from outside to begin with, so I don't think we should imprison it in the pipeline.
• Add nf-test to GTF_FILTER module #1166 gtf_filter/main.nf - (quite custom for this pipeline, leave local)
• Add nf-test to MULTIQC_CUSTOM_BIOTYPE module #1167 multiqc_custom_biotype/main.nf - leave local
• Replace local MULTIQC with nf-core version #1168 multiqc/main.nf - (-> modules)
• Add nf-test to PREPROCESS_TRANSCRIPTS_FASTA_GENCODE module #1169 preprocess_transcripts_fasta_gencode/main.nf -> leave local
• Add nf-test to RSEM_MERGE_COUNTS module #1170 rsem_merge_counts/main.nf -> leave local
• Add nf-test to STAR_ALIGN_IGENOMES module #1175 star_align_igenomes/main.nf -> try to fix it
• Add nf-test to STAR_GENOMEGENERATE_IGENOMES module #1176 star_genomegenerate_igenomes/main.nf -> try to fix it
• Migrate SUMMARIZEDEXPERIMENT to nf-core/modules #1171 summarizedexperiment/main.nf -> nf-core/modules
• Migrate TX2GENE to nf-core/modules #1172 tx2gene/main.nf -> fix and don't use params in the process -> or even maybe see with differentialabundance
• Migrate TXIMPORT to nf-core/modules #1173 tximport/main.nf - (-> modules, probably using R template pattern)
• Investigate replacing UMITOOLS_PREPAREFORRSEM with a more generic version #1174 umitools_prepareforrsem/main.nf > let's check status of script, and nf-core/modules