pytest migration: search: use tmpdir fixture instead of 'current dir'

The test_empty_search method needs to refer to an existing directory that is
guaranteed to not contain a search index. The current implementation chose
the 'current' directory '.' but it is more of a hack than careful planning.
A temporary empty directory would be cleaner but was more involved to
create.

With the introduction of pytest-style test classes, this can very easily
be improved. Creating a temporary directory with pytest is as simple as
accepting the magic 'tmpdir' argument to the test method. This 'tmpdir' is
a fixture provided by pytest. The variable is initialized with a
py.path.local object referring a temporary directory. For details, see:
http://pytest.org/latest/tmpdir.html