CTest: Add test for running many tests in parallel

In particular, this checks that CTest's use of select() has a sufficient
file descriptor set size limit (FD_SETSIZE) to handle many child
processes at the same time.  Running 20 tests requires more than 64
descriptors, the Cygwin default that we override.
Merge topic 'cygwin_fd_setsize' into next

875eb8e CTest: Add test for running many tests in parallel