replace testdata/scripts/*.txt with testdata/script/*.txtar

Following the best practices from upstream.
In particular, the "txt" extension is somewhat ambiguous.

This may cause some conflicts due to the git diff noise,
but hopefully we won't ever do this again.

main_test.go

@@ -53,7 +53,7 @@ func (m garbleMain) Run() int {
 
 var update = flag.Bool("u", false, "update testscript output files")
 
-func TestScripts(t *testing.T) {
+func TestScript(t *testing.T) {
 	t.Parallel()
 
 	execPath, err := os.Executable()
@@ -62,7 +62,7 @@ func TestScripts(t *testing.T) {
 	}
 
 	p := testscript.Params{
-		Dir: filepath.Join("testdata", "scripts"),
+		Dir: filepath.Join("testdata", "script"),
 		Setup: func(env *testscript.Env) error {
 			env.Vars = append(env.Vars,
 				// Use testdata/mod as our module proxy.