common: add DumpCommand check to packerCommand

In addition to dumping the results of a command, we also now dump the
command itself, so we know exactly which command was invoked to produce
the result we got.

packer_test/common/check/gadgets.go

@@ -195,6 +195,20 @@ func (d dump) Check(stdout, stderr string, err error) error {
 	return nil
 }
 
+func DumpCommand(t *testing.T, command string) Checker {
+	return &dumpCommand{t, command}
+}
+
+type dumpCommand struct {
+	t       *testing.T
+	command string
+}
+
+func (d dumpCommand) Check(_, _ string, _ error) error {
+	d.t.Logf("ran command %s", d.command)
+	return nil
+}
+
 type PanicCheck struct{}
 
 func (_ PanicCheck) Check(stdout, stderr string, _ error) error {

packer_test/common/commands.go

@@ -135,6 +135,17 @@ func (pc *packerCommand) Dump() *packerCommand {
 	return pc
 }
 
+func (pc *packerCommand) commandString() string {
+	buf := &strings.Builder{}
+
+	fmt.Fprintf(buf, "%q", pc.packerPath)
+	for _, arg := range pc.args {
+		fmt.Fprintf(buf, " %q", arg)
+	}
+
+	return buf.String()
+}
+
 // Run executes the packer command with the args/env requested and returns the
 // output streams (stdout, stderr)
 //
@@ -194,6 +205,7 @@ func (pc *packerCommand) Output() (string, string, error) {
 func (pc *packerCommand) Assert(checks ...check.Checker) {
 	if pc.dump {
 		tmpChecks := []check.Checker{
+			check.DumpCommand(pc.t, pc.commandString()),
 			check.Dump(pc.t),
 		}