Use blocking reader to fix race in test

The `TestCanceledContext` test in `twofactorverify_test.go` tests
the scenario wherein the request context gets canceled in the
middle of the request.

A race can occur though when the input gets read before the context
is canceled which can result to a different error: instead of
having the context canceled, the OTP will be blank.

To fix it, we use a blocking reader as input to simulate "waiting
for input" scenario. This way, reading the input will never be
finished and the context cancelation can be done appropriately.

internal/command/twofactorverify/twofactorverify_test.go

@@ -177,7 +177,7 @@ func TestCanceledContext(t *testing.T) {
 	cmd := &Command{
 		Config:     &config.Config{GitlabUrl: url},
 		Args:       &commandargs.Shell{GitlabKeyId: "wait_infinitely"},
-		ReadWriter: &readwriter.ReadWriter{Out: output, In: &bytes.Buffer{}},
+		ReadWriter: &readwriter.ReadWriter{Out: output, In: &blockingReader{}},
 	}
 
 	ctx, cancel := context.WithCancel(context.Background())