test: Fix test_fd_limit failures on systems with very high fd limits
test: Fix test_fd_limit failures on systems with very high fd limits

- On newer kernels, the default file descriptor limit is higher than
the number of local TCP ports available for loopback connections,
causing unexpected EADDRNOTAVAIL returns from mrpc_connect().  Set the
test process' fd limit to a fixed value before starting the test.

cmusatyalab/minirpc,cmusatyalab/minirpc