tests: finish parsing and X25519 tests, distinguish HMAC errors

If the implementation re-encodes the header before checking the HMAC,
that would mask malleability issues: the HMAC check would fail because
the tests HMAC'd the original header, but an attacker could also produce
the right HMAC. Instead of duplicating every parsing tests (with the
original and re-encoded HMAC), we make the test framework distinguish
HMAC errors, which ensures bad encodings are recognized as such and not
bypassable HMAC errors.

testdata/testkit/x25519_long_share

@@ -0,0 +1,10 @@
+expect: header failure
+file key: 59454c4c4f57205355424d4152494e45
+identity: AGE-SECRET-KEY-1EGTZVFFV20835NWYV6270LXYVK2VKNX2MMDKWYKLMGR48UAWX40Q2P2LM0
+comment: a trailing zero is missing from the X25519 share
+
+age-encryption.org/v1
+-> X25519 TEiF0ypqr+bpvcqXNyCVJpL7OuwPdVwPL7KQEbFDOCcA
+hjabGXwSLQ9c3S6Lw2i+S2Tu2fiwQHHslbBN6B41FLE
+--- QbEwdWirchS37UUOPh7uVddRiOaWjFwRUpaQ4Q+Z1RE
+îÏbÇΑ´3'NhÔòùL·L[þ÷¾ªRÈð¼™,ƒ1ûf