test/recipes/90-test_store.t: Rename some functions

to_rel_file_uri really treated all files appropriately, absolute and
relative alike, and really just constructs a URI, so gets renamed to
to_file_uri

to_file_uri, on the other hand, forces the path into an absolute one,
so gets renamed to to_abs_file_uri

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3907)