testscript: fix "signal: killed" exec errors

By adding a small sleep before `TestingM.Run()` to allow the write of the test commands to be flushed to disk.

Fixes 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 +98,10 @@ func RunMain(m TestingM, commands map[string]func() int) (exitCode int) {
 				ts.cmdExec(neg, append([]string{name}, args...))
 			}
 		}
+
+		// Make sure all of the io operations above gets some time to complete.
+		time.Sleep(50 * time.Millisecond)
+
 		return m.Run()
 	}
 	// The command being registered is being invoked, so run it, then exit.