tests: Avoid OVN environment variables leaking into tests.

tests: Avoid OVN environment variables leaking into tests.

If $OVN_NB_DB or $OVN_SB_DB happened to be set in the environment in which
"make check" was invoked, then their values would leak into the tests'
environment and interfere with the tests.  This commit avoids that problem.

OVS has a number of environment variables too, such as OVS_RUNDIR, but
the tests already set those to custom values.

Reported-by: Han Zhou <zhouhan@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Han Zhou <hzhou8@ebay.com>