Fixed bug: too many Ns added by spliced_seq

If sequence of an exon is not found, Spliced_seq should add the number
of Ns corresponding to just that exon.
Add tests for spliced_seq remote sequence handling

Add tests to check that the length of spliced_seq is the same when it
resorts to padding with Ns after failing to locate the remote source
sequence for some exons. Also checks that spliced_seq throws the
appropriate warning. The test throws a warning itself if it cannot
complete the tests e.g. due to lack of Genbank access.
Splicedseq.t: Reorganized new tests to use "-requires_networking"
for online tests, and made a format cleaning