support/testing: revert the last change of check_network()

check_network() must check the error code of the command
used to check the network configuration with the value
passed as argument "exitCode".

But this argument is ignored since this commit [1].

Revert the last change of check_network().

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/1522848308
https://gitlab.com/kubu93/buildroot/-/jobs/1522848306



[1] afc1ed4d

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>