selftests: core: add tests for CLOSE_RANGE_CLOEXEC

selftests: core: add tests for CLOSE_RANGE_CLOEXEC

check that close_range(initial_fd, last_fd, CLOSE_RANGE_CLOEXEC)
correctly sets the close-on-exec bit for the specified file
descriptors.

Open 100 file descriptors and set the close-on-exec flag for a subset
of them first, then set it for every file descriptor above 2.  Make
sure RLIMIT_NOFILE doesn't affect the result.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Link: https://lore.kernel.org/r/20201118104746.873084-3-gscrivan@redhat.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>