Lint test `features` tag (#1060)

A recent commit introduced a document that enumerated acceptable values
for the test "features" metadata tag. However, this list was incomplete,
and maintaining it placed extra burden on the project owners.

Restructure the document into a machine-readable format. Add entries for
all previously-omitted values. Add in-line documentation with
recommendations for maintenance of the file. Extend the project's
linting tool to validate tests according to the document's contents.