offer: add a test with expiry info

[vincenzopalazzo]

While reviewing the code in [1], an interesting issue was discussed related to the expiry of the offer.

The reported problem is as follows:

At least locally, if I set the expiry to 1 day (86400), it shows
the same expiry on decoding the offer as expected, but if I try to
pay it, it gives me an InvoiceRequestExpired error. However, if I
do the same with 1000 days, it works fine...

This commit adds a test to ensure that we can pay an offer with an expiry of 1 day without encountering an InvoiceRequestExpired error.

[1] getAlby/hub#1242
Signed-off-by: Vincenzo Palazzo vincenzopalazzodev@gmail.com