LU-17989 tests: Cleanup fake interface usage

LU-17989 tests: Cleanup fake interface usage

Callers of have_interface() are using it to check that setup_fakeif()
completed successfully. Update setup_fakeif() to perform its own
validation. Both IPv4 and IPv6 addresses should be assigned to the
fake interface, so setup_fakeif() now checks for both of these.

Test cases that use the non-default namespace should call
setup_netns()/cleanup_netns() themselves rather than relying on the
test suite to setup this environment.

Test-Parameters: trivial
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I80451c55b8b7b6919b7d7e94e72265f2e6aa2854
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55465
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>