Do not emit warnings from in-language unit tests when compiling with forc build

[mohammadfawaz]

Compiler the standard library emits the following warnings now:

warning
   --> /Users/mohammad/Desktop/fuel/sway/sway-lib-std/src/bytes.sw:664:5
    |
662 |
663 |     assert(bytes.len() == 9);
664 |     bytes.pop();
    |     ----------- This returns a value of type Option<u8>, which is not assigned to anything and is ignored.
665 |     bytes.pop();
666 |     bytes.pop();
    |
____

warning
   --> /Users/mohammad/Desktop/fuel/sway/sway-lib-std/src/bytes.sw:665:5
    |
663 |
664 |     bytes.pop();
665 |     bytes.pop();
    |     ----------- This returns a value of type Option<u8>, which is not assigned to anything and is ignored.
666 |     bytes.pop();
667 |     bytes.pop();
    |
____

warning
   --> /Users/mohammad/Desktop/fuel/sway/sway-lib-std/src/bytes.sw:666:5
    |
664 |
665 |     bytes.pop();
666 |     bytes.pop();
    |     ----------- This returns a value of type Option<u8>, which is not assigned to anything and is ignored.
667 |     bytes.pop();
668 |     bytes.pop();
    |
____

warning
   --> /Users/mohammad/Desktop/fuel/sway/sway-lib-std/src/bytes.sw:667:5
    |
665 |
666 |     bytes.pop();
667 |     bytes.pop();
    |     ----------- This returns a value of type Option<u8>, which is not assigned to anything and is ignored.
668 |     bytes.pop();
669 |     bytes.pop();
    |
____

warning
   --> /Users/mohammad/Desktop/fuel/sway/sway-lib-std/src/bytes.sw:668:5
    |
666 |
667 |     bytes.pop();
668 |     bytes.pop();
    |     ----------- This returns a value of type Option<u8>, which is not assigned to anything and is ignored.
669 |     bytes.pop();
670 |     assert(bytes.len() == 3);
    |
____

warning
   --> /Users/mohammad/Desktop/fuel/sway/sway-lib-std/src/bytes.sw:669:5
    |
667 |
668 |     bytes.pop();
669 |     bytes.pop();
    |     ----------- This returns a value of type Option<u8>, which is not assigned to anything and is ignored.
670 |     assert(bytes.len() == 3);
671 |     assert(bytes.pop().unwrap() == c);
    |
____

  Compiled library "std" with 6 warnings.
  Bytecode size is 28 bytes.

I'm getting rid of the warning in #3587 but we should not be emitting warnings from tests when compiling with forc build.