Code fixes on TCP changes

Crete new test method inside the Sysfs class to change string values using C strings instead of using the std::string. This will enable to use strcmp which is more accurate than string::find, imagine the following scenario:
User input for changing the TCP congression algorithm is west
string::find() finds west in westwood and create_w() method attempts to write that to the sysfs interface, returning true
Using strcmp(), create_w() method will only attempt to write and return true if the strings are strictly equal, fixing the case when one string is a substring of the other.

Fix will be applied to other tunnables too.

Signed off by: Erik Birgiser <ariesrik96@gmail.com>