testscript: use unix.CloneFile on MacOs

To fix unexpected errors of type:

```
[signal: killed]
FAIL: testscripts/myecho.txt:1: unexpected command failure
```

Fixes rogpeppe#200

clonefile.go

@@ -0,0 +1,11 @@
+//go:build unix && !darwin
+// +build unix,!darwin
+
+package testscript
+
+import "os"
+
+// cloneFile creates to as a hard link to the from file.
+func cloneFile(from, to string) error {
+	return os.Link(from, to)
+}

clonefile_darwin.go

@@ -0,0 +1,8 @@
+package testscript
+
+import "golang.org/x/sys/unix"
+
+// cloneFile clones the file from to the file to.
+func cloneFile(from, to string) error {
+	return unix.Clonefile(from, to, 0)
+}

clonefile_other.go

@@ -0,0 +1,11 @@
+//go:build !unix
+// +build !unix
+
+package testscript
+
+import "fmt"
+
+// We don't want to use hard links on Windows, as that can lead to "access denied" errors when removing.
+func cloneFile(from, to string) error {
+	return fmt.Errorf("unavailable")
+}

exe.go

@@ -122,10 +122,8 @@ func RunMain(m TestingM, commands map[string]func() int) (exitCode int) {
 // system's temporary directory, like we do. We don't use hard links on Windows,
 // as that can lead to "access denied" errors when removing.
 func copyBinary(from, to string) error {
-	if runtime.GOOS != "windows" {
-		if err := os.Link(from, to); err == nil {
-			return nil
-		}
+	if err := cloneFile(from, to); err == nil {
+		return nil
 	}
 	writer, err := os.OpenFile(to, os.O_WRONLY|os.O_CREATE, 0o777)
 	if err != nil {

go.mod

@@ -2,4 +2,7 @@ module fortio.org/testscript
 
 go 1.20
 
-require golang.org/x/tools v0.7.0
+require (
+	golang.org/x/sys v0.8.0
+	golang.org/x/tools v0.8.0
+)

go.sum

@@ -1,2 +1,4 @@
-golang.org/x/tools v0.7.0 h1:W4OVu8VVOaIO0yzWMNdepAulS7YfoS3Zabrm8DOXXU4=
-golang.org/x/tools v0.7.0/go.mod h1:4pg6aUX35JBAogB10C9AtvVL+qowtN4pT3CGSQex14s=
+golang.org/x/sys v0.8.0 h1:EBmGv8NaZBZTWvrbjNoL6HVt+IVy3QDQpJs7VRIw3tU=
+golang.org/x/sys v0.8.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/tools v0.8.0 h1:vSDcovVPld282ceKgDimkRSC8kpaH1dgyc9UMzlt84Y=
+golang.org/x/tools v0.8.0/go.mod h1:JxBZ99ISMI5ViVkT1tr6tdNmXeTrcpVSD3vZ1RsRdN4=