Tests: test_session: include tap.h last

tap.h defines a number of macros that are very likely to clash with
other headers (e.g. ok, fail, etc.). On gcc 7.5.0, builds fail whenever
tap.h is included before an header that transitively includes
basic_ios.h.

This clash doesn't occur with more recent gcc releases (tested with 11.2
on my local machine).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I62aaa14a6d1f20c77073ab2e481eddcb28fb78f3