test: sstable_directory_test: make new_sstable() not a template
test: sstable_directory_test: make new_sstable() not a template

new_sstable is defined as a template, and later used in a context
that requires an object. Somehow gcc uses an instantiation with
an empty template parameter list, but I don't think it's right,
and clang refuses.

Since the template is gratuitous anyway, just make it a regular
function.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla