[test] always use a tempdir for server fixture config file

When starting up a Soledad Server through the use of a fixture in tests,
a config file is created with the options needed for that test. As the
config file was being created in /etc and not being removed, this was
influencing servers started by other tests that don't use that fixture.

By always using a temporary directory in the server fixture, we make
sure that the config file is used only for that test and is removed
after the test is finished.

Closes: #8995

tests/conftest.py

@@ -205,16 +205,7 @@ class SoledadServer(object):
         ])
 
     def _create_conf_file(self):
-
-        # come up with name of the configuration file
-        fname = '/etc/soledad/soledad-server.conf'
-        if not os.access('/etc', os.W_OK):
         fname = os.path.join(self.tmpdir.strpath, 'soledad-server.conf')
-
-        # create the configuration file
-        dirname = os.path.dirname(fname)
-        if not os.path.isdir(dirname):
-            os.mkdir(dirname)
         with open(fname, 'w') as f:
             blobs_path = os.path.join(str(self.tmpdir), 'blobs')
             content = '''[soledad-server]