Fix #348: All exercises resolve to complete when one is completed

[jcheng5]

Fixes #348.

---

Run the following, click the "Run Code" buttons one after another quickly. The progress spinner on both code chunks clear after 1 second, even though the two chunk doesn't complete for a few more seconds.

---
title: "Tutorial"
output: learnr::tutorial
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(learnr)
```

```{r one, exercise=TRUE}
Sys.sleep(1); print("DONE!")
```

```{r two, exercise=TRUE}
Sys.sleep(5); print("DONE2!")
```

PR task list:

• Update NEWS
• Add tests (if possible)
• Update documentation with devtools::document()

[jcheng5]

Reviewed by @schloerke in person