test: Reuse macOSAvailable code from Code-Hex/vz

We want to avoid running the macOS13-specific tests on older macOS
versions.

test/osversion.go

@@ -0,0 +1,54 @@
+package test
+
+// The code in this file is heavily based on https://github.com/Code-Hex/vz/blob/40946b951fffa07406b272c582eda76de7c24028/osversion.go#L46-L70
+
+import (
+	"errors"
+	"strconv"
+	"strings"
+	"sync"
+	"syscall"
+
+	"golang.org/x/mod/semver"
+)
+
+func macOSAvailable(version float64) error {
+	if macOSMajorMinorVersion() < version {
+		return ErrUnsupportedOSVersion
+	}
+	return nil
+}
+
+var (
+	// ErrUnsupportedOSVersion is returned when calling a method which is only
+	// available in newer macOS versions.
+	ErrUnsupportedOSVersion = errors.New("unsupported macOS version")
+
+	majorMinorVersion     float64
+	majorMinorVersionOnce interface{ Do(func()) } = &sync.Once{}
+)
+
+func fetchMajorMinorVersion() (float64, error) {
+	osver, err := syscall.Sysctl("kern.osproductversion")
+	if err != nil {
+		return 0, err
+	}
+	prefix := "v"
+	majorMinor := strings.TrimPrefix(semver.MajorMinor(prefix+osver), prefix)
+	version, err := strconv.ParseFloat(majorMinor, 64)
+	if err != nil {
+		return 0, err
+	}
+	return version, nil
+}
+
+func macOSMajorMinorVersion() float64 {
+	majorMinorVersionOnce.Do(func() {
+		version, err := fetchMajorMinorVersion()
+		if err != nil {
+			panic(err)
+		}
+		majorMinorVersion = version
+	})
+	return majorMinorVersion
+}