testscript: fix "signal: killed" exec errors on MacOS

On `MacOS` there are lots of reports of unexpected failing tests with output similar to this:

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

On CI builds the workaround has been to downgrade to a builder with MacOS <= 11 (e.g. macos-11 on GitHub).

In development on a MacBook, this is not an option.

This commit works around what seem to be a upstream bug in `os.Link` by adding a small sleep before `TestingM.Run()` to allow the write of the test
command symlinks to be ready.

See rogpeppe#200

testscript/exe.go

@@ -12,6 +12,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"time"
 )
 
 // TestingM is implemented by *testing.M. It's defined as an interface
@@ -97,6 +98,13 @@ func RunMain(m TestingM, commands map[string]func() int) (exitCode int) {
 				ts.cmdExec(neg, append([]string{name}, args...))
 			}
 		}
+
+		if runtime.GOOS == "darwin" {
+			// Make sure all of the io operations above gets some time to complete.
+			// See issue #200.
+			time.Sleep(200 * time.Millisecond)
+		}
+
 		return m.Run()
 	}
 	// The command being registered is being invoked, so run it, then exit.