The files t/spectest.data* specify the list of files to use to test a
specific roast version. The version to test is obtained from the VERSION
file in roast checkout (t/spec/VERSION).
The default file is t/spectest.data and it's used if roast version could not
be obtained, if the spectest.data file for the requested version doesn't
exist or isn't readable, or if the version matches string propo (e.g. 6.d.proposal).
Otherwise, the version is used as a suffix, separated with a dot:
VERSION file contains "6.c" => tests read from t/spectest.data.6.c
The master roast branch would typically contain a proposal version (6.d.proposal).
Once that language version is released and a new branch with it is published, the
VERSION file will be changed (6.d) and a new spectest.data file will be created
(t/spectest.data.6.d).