builder/docker: tests for some exec stuff

builder/docker/exec_test.go

+package docker
+
+import (
+	"testing"
+)
+
+func TestCleanLine(t *testing.T) {
+	cases := []struct {
+		input  string
+		output string
+	}{
+		{
+			"\x1b[0A\x1b[2K\r8dbd9e392a96: Pulling image (precise) from ubuntu\r\x1b[0B\x1b[1A\x1b[2K\r8dbd9e392a96: Pulling image (precise) from ubuntu, endpoint: https://cdn-registry-1.docker.io/v1/\r\x1b[1B",
+			"8dbd9e392a96: Pulling image (precise) from ubuntu, endpoint: https://cdn-registry-1.docker.io/v1/",
+		},
+	}
+
+	for _, tc := range cases {
+		actual := cleanOutputLine(tc.input)
+		if actual != tc.output {
+			t.Fatalf("bad: %#v %#v", tc.input, actual)
+		}
+	}
+}