test 518 is all about testing libcurl functionality

when more than FD_SETSIZE file descriptors are open.
This means that if for any reason we are not able to
open more than FD_SETSIZE file descriptors then test
518 should not be run.

test 537 is all about testing libcurl functionality
when the system has nearly exhausted the number of
free file descriptors. Test 537 will try to run with
very few free file descriptors.