settle for only running the ulimit test on linux

I tried using the unix build tag, although predictably,
the test is prone to failure on darwin:

	fork/exec /var/[...]/gofumpt.test: argument list too long

Testing ulimits on one GOOS is enough,
especially since we'll always test on linux via CI.

ulimit_unix_test.go → ulimit_linux_test.go

@@ -1,10 +1,6 @@
 // Copyright (c) 2019, Daniel Martí <mvdan@mvdan.cc>
 // See LICENSE for licensing information
 
-// TODO: replace with the unix build tag once we require Go 1.19 or later
-//go:build linux
-// +build linux
-
 package main
 
 import (