test: reduce further influence from the environment

test: reduce further influence from the environment

Some tests used the default home directory which can have side-effects
(such as loading plugins, loading deprecated preferences). These could
cause tests to fail. Always use a sane environment to fix this.

Change getTsharkInfo to use this clean environment as well
(WIRESHARK_CONFIG_DIR does not exist with master-2.6 and would also not
propagate things like ASAN_OPTIONS=detect_leaks=0).

Change-Id: I1674f71972d35de91d191e0c29fdb59b8a0a56ce
Reviewed-on: https://code.wireshark.org/review/30165
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>