Test framework: Add the possibility to have a test specific data dir

This data directory is formed automatically by taking the recipe name
and changing '.t' to '_data'.  Files in there can be reached with the
new function data_file()

(Merged from https://github.com/openssl/openssl/pull/2027)
(cherry picked from commit 6c6a2ae6

)
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2802)