TestClientNotificationStorm is broken on FreeBSD

[gballet]

System information

Geth version: cffdc43954c5f5fa4364ddfbf3881cdfeb667fb7
OS & Version: FreeBSD

Expected behaviour

TestClientNotificationStorm passes on FreeBSD

Actual behaviour

> go run build/ci.go test -coverage ./rpc/
>>> /usr/local/go/bin/go test -ldflags -X main.gitCommit=cffdc43954c5f5fa4364ddfbf3881cdfeb667fb7 -X main.gitDate=20210211 -p 1 -covermode=atomic -cover ./rpc/
--- FAIL: TestClientNotificationStorm (2.40s)
    client_test.go:425: didn't get expected error
FAIL
coverage: 84.8% of statements
FAIL	github.com/ethereum/go-ethereum/rpc	6.768s
FAIL
util.go:47: exit status 1
exit status 1

Steps to reproduce the behaviour

go run build/ci.go test -coverage ./rpc/

[gballet]

Looks like FreeBSD is able to handle more subscription than other OSes, I can fix the test by setting the limit to 50k in client_test.go.