Added basic unit testing capabilities for the server

*Added tests.c and tests.h that execute the unit tests
*Added test_login.c unit test for the login.c server source file
*Added tests directory in bin for the created test executable
*Fixed the login.c _check_credentials function by removing
 the newline character at the end of the read line from users.txt
*Added server_tests target to the Makefile along with adding variables and commands