Add initial support for test cases

[gadenbuie]

Adds initial support for test cases, which can be added to a <exercise-label>-test-cases chunk. The test cases can be named and split using code sections, e.g. # first test ----. They are added to the exercise object under the test_cases item, which is either NULL or a list of test cases. If there are no code sections, the single test cases will be in an unnamed list, otherwise test cases are named with default names test_case_NN where NN is a left-zero-padded integer.

I've included a manual test in tests/manual/test-cases.Rmd

Example Rmd code

```{r addition-setup}
x <- 1 + 1
```

```{r addition, exercise = TRUE}

```

```{r addition-solution}
1 + 1
```

```{r addition-check}
check_this_exercise(user_code, solution_code)
```

```{r addition-test-cases}
1 + 1

# one plus two ----
1 + 2

# one plus three ----
1 + 3

# one equals three ----
1 = 3

# 2 minus one ----
2 - 1
```

pkgload::load_all()

x <- prepare_tutorial_cache_from_source("tests/manual/test-cases.Rmd")

str(x$items$data[[1]][c("solution", "check", "test_cases")])
#> List of 3
#>  $ solution  : chr "1 + 1"
#>   ..- attr(*, "chunk_opts")=List of 1
#>   .. ..$ label: chr "addition-solution"
#>  $ check     : chr "check_this_exercise(user_code, solution_code)"
#>   ..- attr(*, "chunk_opts")=List of 1
#>   .. ..$ label: chr "addition-check"
#>  $ test_cases:List of 5
#>   ..$ test_case_00    : chr "1 + 1"
#>   ..$ one plus two    : chr "1 + 2"
#>   ..$ one plus three  : chr "1 + 3"
#>   ..$ one equals three: chr "1 = 3"
#>   ..$ 2 minus one     : chr "2 - 1"

For now, this initial support only ensures that -test-cases chunks are removed from the tutorial presentation and are stored alongside the exercise object. In the future, we hope to expand test case support, but we want to gather some experience with this format and structure first.

[rossellhayes]

This looks like a great first step. Will test case code and expected results all be stored together in the single chunk? We'll have to think of an interface for including that

[gadenbuie]

Will test case code and expected results all be stored together in the single chunk? We'll have to think of an interface for including that

The test cases can be arbitrary code, so for now it can be whatever we need it to be. I was originally thinking it would be examples of student submissions but it could be testing code or anything else really that would become part of the exercise data.

Now that I've written out the above, I think I'm now leaning more toward -tests for the chunk name, e.g. addition-tests, with the object name $tests and individual items $tests$test01, etc. -test-cases is a bit more specific than the feature might end up being.

[rossellhayes]

Got it! Would it be implemented with a testthat-style interface (i.e. all tests must not produce an error)?

[gadenbuie]

@rossellhayes I just changed the name from -test-cases to -tests and updated the structure accordingly. Because this overlaps considerably with rstudio/gradethis#286, I moved the main functions in this PR, plus a few others into inst/staticexports and started using staticimports. The plan is to statically import these functions in gradethis too once this PR merges. This sets up learnr as the parent location for these kinds of helper/utility functions that we want to use across packages.