syscall: TestOpenFileLimit unintentionally runs on non-Unix platforms

[dmitshur]

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, the test only applies on Unix too.
But it currently runs on all platforms.

In particular, this causes a problem where TestOpenFileLimit is
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.

[dmitshur]

This is fixed on tip via CL 539435.

@gopherbot Would you please create a backport issue for Go 1.21?
This is a test-only fix needed to fix trybots on the release branch.

[gopherbot]

Backport issue(s) opened: #63994 (for 1.21).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.