testscript: support stdout/stderr args in stdin (#94)

This is particularly useful to "pipe" output like "stdin stdout". A
simple test is added.

testscript/cmd.go

@@ -363,9 +363,7 @@ 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.

testscript/doc.go

@@ -180,6 +180,8 @@ 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

testscript/testdata/stdin.txt

@@ -4,6 +4,15 @@ stdout hello
 [exec:cat] exec cat
 ! stdout hello
 
+
+[!exec:cat] stop
+
+# Check that 'stdin stdout' works.
+exec cat hello.txt
+stdin stdout
+exec cat
+stdout hello
+
 -- hello.txt --
 hello