syscall: TestOpenFileLimit unintentionally runs on non-Unix platforms [1.21 backport]

[gopherbot]

@dmitshur requested issue #63993 to be considered for backport to the next 1.21 minor release.

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]

Change https://go.dev/cl/540615 mentions this issue: [release-branch.go1.21] syscall: copy rlimit.go's build constraint to rlimit_test.go

[gopherbot]

Closed by merging 1e91861 to release-branch.go1.21.