ui, termstatus: Move WrapStdio

Saves some imports. ui still needs to import ui/termstatus from
message.go.

internal/ui/termstatus/stdio_wrapper_test.go

@@ -0,0 +1,84 @@
+package termstatus
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestStdioWrapper(t *testing.T) {
+	var tests = []struct {
+		inputs [][]byte
+		output string
+	}{
+		{
+			inputs: [][]byte{
+				[]byte("foo"),
+			},
+			output: "foo\n",
+		},
+		{
+			inputs: [][]byte{
+				[]byte("foo"),
+				[]byte("bar"),
+				[]byte("\n"),
+				[]byte("baz"),
+			},
+			output: "foobar\n" +
+				"baz\n",
+		},
+		{
+			inputs: [][]byte{
+				[]byte("foo"),
+				[]byte("bar\nbaz\n"),
+				[]byte("bump\n"),
+			},
+			output: "foobar\n" +
+				"baz\n" +
+				"bump\n",
+		},
+		{
+			inputs: [][]byte{
+				[]byte("foo"),
+				[]byte("bar\nbaz\n"),
+				[]byte("bum"),
+				[]byte("p\nx"),
+				[]byte("x"),
+				[]byte("x"),
+				[]byte("z"),
+			},
+			output: "foobar\n" +
+				"baz\n" +
+				"bump\n" +
+				"xxxz\n",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run("", func(t *testing.T) {
+			var output strings.Builder
+			w := newLineWriter(func(s string) { output.WriteString(s) })
+
+			for _, data := range test.inputs {
+				n, err := w.Write(data)
+				if err != nil {
+					t.Fatal(err)
+				}
+
+				if n != len(data) {
+					t.Errorf("invalid length returned by Write, want %d, got %d", len(data), n)
+				}
+			}
+
+			err := w.Close()
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if outstr := output.String(); outstr != test.output {
+				t.Error(cmp.Diff(test.output, outstr))
+			}
+		})
+	}
+}