Add: test to ensure correct swapping of the magic number

[laustam]

I am suggesting this test addition as a result of a mistake made by one of my students.

Their implementation went as follows:

uint8_t magic_num_buf[4];
fread(magic_num_buf, sizeof(byte_t), 4, fp);
uint32_t magic_num = read_uint32(magic_num_buf);

if (!(magic_num == MAGIC_NUMBER || swap_uint32(magic_num) == MAGIC_NUMBER))
  // not an ijvm file
  return NULL;
}

The problematic part here is that the student does not fully understand the notion of endianness; they check whether the unswapped or swapped version of the first four bytes equals MAGIC_NUMBER (0x1deadfad) to determine whether the file is a valid IJVM binary. This is incorrect: files starting with 0xaddfea1d are not valid IJVM binaries.

To address this misconception, I propose a test, magicnum_swap, which asserts that initializing an IJVM with a given file that starts with 0xaddfea1d returns NULL (ie. the initialization fails).