Use Process.getrlimit instead of `ulimit` in the tests.

In modern shells ulimit is a builtin so calling it within backticks can use odd
file not found errors (below). Process.getrlimit should be a safe replacement.

```
  1) Error:
test_fd_gc(TestParser):
Errno::ENOENT: No such file or directory - ulimit -n
    ~/projects/oss/libxml-ruby/test/tc_parser.rb:249:in ``'
    ~/projects/oss/libxml-ruby/test/tc_parser.rb:249:in `test_fd_gc'
```