A fair bit of setup / teardown code was being duplicated in many test cases in the socketpair testsuite. Take advantage of the `setup()`, `before()`, and `after()` functions in the new ZTest API. Signed-off-by: Christopher Friedt <cfriedt@meta.com>
Add a test to exercise `zsock_ioctl(fd, ZFD_IOCTL_FIONREAD, &avail)`. Signed-off-by: Christopher Friedt <cfriedt@meta.com>