packer_test: dump command outs in case of failure

When a test fails to exert its assertions on the command-line output, a
test fails, but we don't necessarily can troubleshoot what happened,
especially when this happens in a CI environment.

Therefore, for convenience, we add the faculty for packerCommand.Assert
to automatically dump a command's output (both stdout and stderr) if a
test fails.

packer_test/commands_test.go

@@ -155,6 +155,10 @@ func (pc *packerCommand) Assert(checks ...Checker) {
 
 		if pc.t.Failed() {
 			pc.t.Errorf("attempt %d failed validation", attempt)
+
+			pc.t.Logf("dumping stdout: %s", stdout)
+			pc.t.Logf("dumping stdout: %s", stderr)
+
 			break
 		}
 	}