testscript: support stdout/stderr in more builtins

Support these special files in the stdin, unquote, and match (grep)
commands. This is particularly useful to "pipe" output like "stdin
stdout", or to use commands that produce txtar output.

The grep one isn't super useful, as one can already do "stdout xxx"
instead of "grep xxx stdout", but this way it's more consistent and we
save some repeated lines. Because the feature is itself a bit redundant,
don't document it, to prevent confusing the user.

testscript/cmd.go

@@ -166,6 +166,7 @@ func (ts *TestScript) cmdCp(neg bool, args []string) {
 			data []byte
 			mode os.FileMode
 		)
+		// Don't use TestScript.ReadFile, as we want the mode too.
 		switch arg {
 		case "stdout":
 			src = arg
@@ -310,9 +311,7 @@ func (ts *TestScript) cmdUnquote(neg bool, args []string) {
 	}
 	for _, arg := range args {
 		file := ts.MkAbs(arg)
-		data, err := ioutil.ReadFile(file)
-		ts.Check(err)
-		data, err = txtar.Unquote(data)
+		data, err := txtar.Unquote([]byte(ts.ReadFile(file)))
 		ts.Check(err)
 		err = ioutil.WriteFile(file, data, 0666)
 		ts.Check(err)
@@ -363,9 +362,7 @@ func (ts *TestScript) cmdStdin(neg bool, args []string) {
 	if len(args) != 1 {
 		ts.Fatalf("usage: stdin filename")
 	}
-	data, err := ioutil.ReadFile(ts.MkAbs(args[0]))
-	ts.Check(err)
-	ts.stdin = string(data)
+	ts.stdin = ts.ReadFile(args[0])
 }
 
 // stdout checks that the last go command standard output matches a regexp.
@@ -494,9 +491,7 @@ func scriptMatch(ts *TestScript, neg bool, args []string, text, name string) {
 	isGrep := name == "grep"
 	if isGrep {
 		name = args[1] // for error messages
-		data, err := ioutil.ReadFile(ts.MkAbs(args[1]))
-		ts.Check(err)
-		text = string(data)
+		text = ts.ReadFile(args[1])
 	}
 
 	if neg {

testscript/doc.go

@@ -171,6 +171,8 @@ The predefined commands are:
   each line. This enables a file to contain substrings that look like
   txtar file markers.
   See also https://godoc.org/github.com/rogpeppe/go-internal/txtar#Unquote
+  File can be "stdout" or "stderr" to use the standard output or standard error
+  from the most recent exec or wait command.
 
 - rm file...
   Remove the listed files or directories.
@@ -180,6 +182,8 @@ The predefined commands are:
 
 - stdin file
   Set the standard input for the next exec command to the contents of the given file.
+  File can be "stdout" or "stderr" to use the standard output or standard error
+  from the most recent exec or wait command.
 
 - [!] stderr [-count=N] pattern
   Apply the grep command (see above) to the standard error