unix: test returned fd in TestEpoll
unix: test returned fd in TestEpoll

Check the fd returned in EpollEvent to detect potential padding issues.

Also, fail the test if the number of events mismatches.

Updates golang/go#35479

Change-Id: I39f856ca2c336e849876a33acffb70b82aa83c3f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/206858
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

multiplay/golang-sys,golang/sys,golang/sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys