os: use a lower file count for TestOpenFileLimit on openbsd

OpenBSD has a default soft limit of 512 and hard limit of 1024 - as such,
attempting to open 1200 files is always going to fail unless the defaults
have been changed. On this platform use 768 instead such that it passes
without requiring customisation.

Fixes #51713

Change-Id: I7679c8fd73d4b263145129e9308afdb29d67bb54
Reviewed-on: https://go-review.googlesource.com/c/go/+/401594
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: 谢致邦 <xiezhibang@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>

src/os/rlimit_test.go

@@ -11,18 +11,21 @@ import (
 )
 
 func TestOpenFileLimit(t *testing.T) {
-	if runtime.GOOS == "openbsd" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64" || runtime.GOARCH == "mips64") {
-		t.Skip("broken on openbsd/arm, openbsd/arm64, openbsd/mips64 builder - go.dev/issue/51713")
-	}
-
 	// For open file count,
 	// macOS sets the default soft limit to 256 and no hard limit.
 	// CentOS and Fedora set the default soft limit to 1024,
 	// with hard limits of 4096 and 524288, respectively.
 	// Check that we can open 1200 files, which proves
 	// that the rlimit is being raised appropriately on those systems.
+	fileCount := 1200
+
+	// OpenBSD has a default soft limit of 512 and hard limit of 1024.
+	if runtime.GOOS == "openbsd" {
+		fileCount = 768
+	}
+
 	var files []*File
-	for i := 0; i < 1200; i++ {
+	for i := 0; i < fileCount; i++ {
 		f, err := Open("rlimit.go")
 		if err != nil {
 			t.Error(err)