[release-branch.go1.21] syscall: copy rlimit.go's build constraint to rlimit_test.go

Tests in rlimit_test.go exist to test the behavior of automatically
bumping RLIMIT_NOFILE on Unix implemented in rlimit.go (issue #46279),
with darwin-specific behavior split out into rlimit_darwin.go and
the rest left empty in rlimit_stub.go.

Since the behavior happens only on Unix, it doesn't make sense to test
it on other platforms. Copy rlimit.go's 'unix' build constraint to
rlimit_test.go to accomplish that.

Leave out the simplification of the build constraint in rlimit_stub.go
so that this CL remains a test-only fix.

In particular, this fixes a problem where TestOpenFileLimit was
failing in some environments when testing the wasip1/wasm port.
The RLIMIT_NOFILE bumping behavior isn't implemented there, so
the test was testing the environment and not the Go project.

Updates #46279.
For #61116.
Fixes #63994.

Change-Id: Ic993f9cfc021d4cda4fe3d7fed8e2e180f78a2ca
Cq-Include-Trybots: luci.golang.try:go1.21-wasip1-wasm_wasmtime
Reviewed-on: https://go-review.googlesource.com/c/go/+/539435
Reviewed-by: Johan Brandhorst-Satzkorn <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
(cherry picked from commit b7cbcf0)
Reviewed-on: https://go-review.googlesource.com/c/go/+/540615
Reviewed-by: Heschi Kreinick <[email protected]>
Auto-Submit: Heschi Kreinick <[email protected]>

Author: dmitshur

src/syscall/rlimit_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build unix
+
 package syscall_test
 
 import (