test: adapt tests for SOURCE_DATE_EPOCH. From https://lists.gnu.org/archive/html/automake/2025-06/msg00016.html. * t/mdate5.sh: allow years 19xx for old SOURCE_DATE_EPOCH. * t/txinfo-vtexi3.sh: likewise. * t/txinfo-vtexi4.sh: if SDE is set, use mdate-sh to parse it into the expected UPDATED and UPDATED-MONTH values. Report from Frederic Berat. * NEWS: mention this.