[tests] Disable MonoTests.Mono.Unix.Native.MemfdTest.TestMemfd test (#4869)

Android 11/API-30 made the [**memfd_create**(2)][0] Linux-specific API
fail whenever the `MFD_CLOEXEC | MFD_ALLOW_SEALING` combination of flags
is passed to it, resulting in "BCL" unit test failures:

	Xamarin.Android.Bcl_Tests, MonoTests.Mono.Unix.Native.MemfdTest.TestMemfd / Release

	System.ArgumentOutOfRangeException : Current platform doesn't support this value.
	Parameter name: value
	Actual value was MFD_CLOEXEC, MFD_ALLOW_SEALING.

Disable the test altogether, since this API isn't used by
`Xamarin.Android` and any applications using it will/should check the
`errno` variable for the `ENOSYS` value to see if the API is supported
or not.  This will unblock us, allowing update of our Android emulator
system image to API 30.

[0]: https://man7.org/linux/man-pages/man2/memfd_create.2.html

Author: grendello

tests/BCL-Tests/Xamarin.Android.Bcl-Tests/nunit-excluded-tests.txt

@@ -12,6 +12,9 @@
 # accept4(), which was added to API-21.
 MonoTests.Mono.Unix.Native.SocketTest.Accept4
 
+# Android 11 (API 30) broke the API for apps targetting API 30
+MonoTests.Mono.Unix.Native.MemfdTest.TestMemfd
+
 # No idea why these no longer throw:
 MonoTests.Mono.Unix.Native.SocketTest.BindConnect
 MonoTests.Mono.Unix.Native.SocketTest.SendMsgRecvMsgAddress