test: set test-fs-cp as flaky

PR-URL: #56799
Refs: #56794
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: StefanStojanovic

test/parallel/parallel.status

@@ -33,6 +33,8 @@ test-runner-watch-mode-complex: PASS, FLAKY
 test-async-context-frame: PASS, FLAKY
 # https://github.com/nodejs/node/issues/54534
 test-runner-run-watch: PASS, FLAKY
+# https://github.com/nodejs/node/issues/56794
+test-fs-cp: PASS, FLAKY
 
 # https://github.com/nodejs/node/issues/56751
 test-without-async-context-frame: PASS, FLAKY