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

@@ -5,7 +5,7 @@ package testscript
 
 import "os"
 
-// cloneFile makes a clone of a file via a hard link.
+// 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

@@ -2,7 +2,7 @@ package testscript
 
 import "golang.org/x/sys/unix"
 
-// cloneFile makes a clone of a file via MacOS's `clonefile` syscall.
+// cloneFile clones the file from to the file to.
 func cloneFile(from, to string) error {
 	return unix.Clonefile(from, to, 0)
 }

clonefile_other.go

@@ -5,9 +5,7 @@ package testscript
 
 import "fmt"
 
-// cloneFile does not attempt anything on Windows, as hard links on it have
-// led to "access denied" errors when deleting files at the end of a test.
-// We haven't tested platforms like plan9 or wasm/wasi.
+// 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")
 }

go.mod

@@ -3,6 +3,6 @@ module fortio.org/testscript
 go 1.20
 
 require (
-	golang.org/x/sys v0.8.0
-	golang.org/x/tools v0.8.0
+	golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f
+	golang.org/x/tools v0.1.12
 )

go.sum

@@ -1,4 +1,4 @@
-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=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f h1:v4INt8xihDGvnrfjMDVXGxw9wrfxYyCjk0KbXjhR55s=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/tools v0.1.12 h1:VveCTK38A2rkS8ZqFY25HIDFscX5X9OoEhJd3quQmXU=
+golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=