Improvement in calculating test ages

[yanj-github]

Curent subsetting script determin Recent and Established tests are based on from Publication or snapshot date for WAMS specification.
Test ages would be better assessed relative to the date of each specification referenced from WMAS, rather than relative to a single cut-off date:
e.g. WMAS 2021 references CSS3 Animations Level 1 from October 2018 so “recent” tests for that spec could be considered to be those added since a year or two before that date, rather than the 1-2 years leading up to the publication of WMAS 2021.