Add test for herb_read_file

[citizen428]

Document that the function exits when the filename is invalid.

Rationale:
I was originally a bit concerned when reading main.c that there was no NULL check around this line:

char* source = herb_read_file(argv[2]);

However, argv[2] should never be empty due to this check:

if (argc < 3) {
  printf("Please specify input file.\n");
  return 1;
}

This leaves the potential problem of non-existent files, which is handled directly inside herb_read_file. This behavior surprised me a little bit, so I thought it might be a good idea to document it with a test.

[citizen428]

Note: I needed to add prism as a dependency of the test target, otherwise make test could not complete after a make clean due to a missing Prism header.

[marcoroth]

Thank you @citizen428! 🙏🏼