[TEST] Add tests for edgecases

[eldariont]

We should add tests for at least the following edge-cases:

• SVs located at the very beginning of a chromosome
• SVs located at the very end of a chromosome (need the genome as input to check that)
• DELs that delete the entire chromosome

For these edge-cases we need to confirm that iGenVar:

• does not crash
• produces valid VCF
• respects the specification (e.g. choosing the right padding base as described here:
  For simple insertions and deletions in which either the REF or one of the ALT alleles would otherwise be null/empty, the REF and ALT Strings must include the base before the event (which must be reflected in the POS field), unless the event occurs at position 1 on the contig in which case it must include the base after the event)