altsvc: only accept 17 byte dates from files

altsvc: only accept 17 byte dates from files

Since the date format is fixed there is no need to accept more data.

Update test355 to verify reject of too long date in alt-svc file

This test case was originally supposed to verify alt-svc loading from a
file but never did because it was done incorrectly.

Now it verifies that a too long date in the input file makes curl
disregard the entry.

Closes #20259