reftable/reader: inline `init_reader()`

reftable/reader: inline `init_reader()`

Most users use an allocated version of the `reftable_reader`, except for
some tests. We are about to convert the reader to become refcounted
though, and providing the ability to keep a reader on the stack makes
this conversion harder than necessary.

Update the tests to use `reftable_reader_new()` instead to prepare for
this change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>