Tests: Fix -Wunreachable-code warnings from clang

Author: https://github.com/nico
Commit: 96666f3209
Pull-request: https://github.com/SerenityOS/serenity/pull/10399

Tests/Kernel/elf-execve-mmap-race.cpp

@@ -123,7 +123,6 @@ int main()
                 hax = !hax;
                 usleep(1);
             }
-            return nullptr;
         },
         &writable_program_headers[0]);
 
@@ -139,5 +138,4 @@ int main()
         printf("waitpid\n");
         waitpid(-1, nullptr, 0);
     }
-    return 0;
 }