test: don't rely on __del__ in test-devel-warnings.t
Whatever the future of __del__ in Mercurial is, that devel-warning test is not
about testing the automatic transaction rollback and we should explicitly call
release.
This change make this tests pass with pypy, as pypy try less hard to call
__del__ at program exit.
[hgk]
path = /usr/share/mercurial/hgk