Replace `create_learnr_env()` in test code with `mock_this_exercise()`

[gadenbuie]

https://github.com/rstudio/gradethis/blob/main/tests/testthat/helper-expect.R#L52-L54

https://github.com/rstudio/gradethis/blob/main/tests/testthat/helper-expect.R#L67-L69

https://github.com/rstudio/gradethis/blob/main/tests/testthat/helper-expect.R#L88-L90