fix: substitute are_equal with == in xargs-test

[Pablo1107]

Basado en esto, habria que hacer lo mismo en los tests de xarg. De lo contrario sucede esto:

Traceback (most recent call last):
  File "/fork/./tests/xargs-test.py", line 9, in <module>
    from utils import VALGRIND_COMMAND, are_equal, color, format_result, run_command
ImportError: cannot import name 'are_equal' from 'utils' (/fork/tests/utils.py)

[Pablo1107]

COMMAND: xargs
packaging arguments [ARGS IN PACKAGE: 4]
  [ARGS SENT] - less: FAIL

Expected:
--------

arg[1]: arg0
arg[2]: arg1
arg[3]: arg2

Got:
---

arg[1]: arg0
arg[2]: arg1
arg[0]: ./tests/argcounter.py
arg[3]: arg2

Al margen del error al ejecutarlo tambien note que no permite que el argumento argv[0] sea el filename que se esta ejecutando, que entiendo segun man 2 execvp deberia ser asi.

v - execv(), execvp(), execvpe()
       The char *const argv[] argument is an array of pointers to null-terminated strings that represent the argument list available
       to the new program.  The first argument, by convention, should point to the filename associated with the file being executed.
       The array of pointers must be terminated by a null pointer.