test: mark test_buffer/test_finalizer flaky

Refs: nodejs/node#43389

Mark new test as flaky as it seems to have
had intermittent failures since it was added.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: nodejs/node#43414
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>

test/node-api/node-api.status

@@ -5,5 +5,6 @@ prefix node-api
 # sample-test                        : PASS,FLAKY
 
 [true] # This section applies to all platforms
+test_buffer/test_finalizer : PASS,FLAKY
 
 [$system==win32]