adding tests for modules/linux_sysctl.py
change linux_sysctl.py to fail properly on misassign

The original check (4e1c367) for sysctl assign failure queries the
format returned by sysctl on stdout.  It was added evidently for some
container system called Virtuozzo.  A later commit (4e1c367) strengthens
the check into a full regex on sysctl stdout.

Unfortunately, it appears that on a contemporary conventional distro,
CentOS 7, sysctl repeats the config it is given on stdout whether or not
the config requested is valid.  Furthermore, an invalid config request
will not cause sysctl to return a nonzero status.  It does print an
error on stderr, so I've added an optional additional check of stderr.
Examples:

centos-7 # sysctl net.ipv4.ip_forward=1 ; echo $?
net.ipv4.ip_forward = 1
0

centos-7 # sysctl net.ipv4.ip_forward=backward ; echo $?
sysctl: setting key "net.ipv4.ip_forward": Invalid argument
net.ipv4.ip_forward = backward
0
remove redundant mock_open() in darwin_sysctl_test.py
pylint fix on linux_sysctl_test.py
Merge pull request #17073 from jfindlay/jmoney/linux_sysctl_test

Jmoney/linux_sysctl_test