testscript: add Defer method (#50)

This allows commands and setup code to add actions that will
be performed at the end of the test. This allows us to (for example)
start a network server in the setup phase of a script and shut
it down afterwards.

Also rename the "scripts" directory to the more conventional "testdata".

Author: rogpeppe

testscript/doc.go

@@ -10,7 +10,7 @@ To invoke the tests, call testscript.Run. For example:
 
 	func TestFoo(t *testing.T) {
 		testscript.Run(t, testscript.Params{
-			Dir: "scripts",
+			Dir: "testdata",
 		})
 	}
 

testscript/scripts/command.txt renamed to testscript/testdata/command.txt

@@ -0,0 +1,3 @@
+testdefer
+testdefer
+testdefer