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.

Also simplify the build constraint in rlimit_stub.go while here,
so that its maintenance is easier and it starts to match all
non-darwin Unix GOOS values (previously, 'hurd' happened to be missed).

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.

Change-Id: Ic993f9cfc021d4cda4fe3d7fed8e2e180f78a2ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/539435
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

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