Merge #10679: Document the non-DER-conformance of one test in tx_valid.json.

Summary:
ecb11f5 Document the non-strict-DER-conformance of one test in tx_valid.json. (Andreas Schildbach)

Tree-SHA512: 4d5ba4645fbfe8fe3f1baaa5f1a1152cdd2cbf3d901f38d8e7fbd56b16caa6a8a17f2a48c74fb725ce454dd1c870b81b2238e89d0639fcd4eee858554726e996

Backport of Core PR10679
https://github.com/bitcoin/bitcoin/pull/10679/files

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3222