test: mark fs-readfile-tostring-fail flaky for all

test-fs-readfile-tostring-fail is unreliable until a libuv fix lands.

It had previously been marked flaky on macOS, but it has been observed
to also fail on AIX. Mark it flaky everywhere.

Refs: libuv/libuv#1742

PR-URL: #21177
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>

test/sequential/sequential.status

@@ -5,6 +5,9 @@ prefix sequential
 # sample-test                       : PASS,FLAKY
 
 [true] # This section applies to all platforms
+# This test will be flaky until https://github.com/libuv/libuv/pull/1742 lands
+# on Node.js.
+test-fs-readfile-tostring-fail: PASS, FLAKY
 
 [$system==win32]
 test-inspector-async-call-stack       : PASS, FLAKY
@@ -15,9 +18,6 @@ test-inspector-async-hook-setup-at-signal:  PASS, FLAKY
 [$system==linux]
 
 [$system==macos]
-# This test will be flaky until https://github.com/libuv/libuv/pull/1742 lands
-# on Node.js.
-test-fs-readfile-tostring-fail: PASS, FLAKY
 
 [$system==solaris] # Also applies to SmartOS