Use envir_prep for exercise checking

[schloerke]

Fixes https://github.com/rstudio/learnr/issues/202

The next release of learnr (v0.10.0) will provide a copy of the environment before the user executed their code envir_prep to the exercise checker function (ex: testwhat_learnr).

Using this environment, checking code will be able to find their setup code.

Disclaimers:

• I have not tested this.
• I do not know if I have broken run_until_fail.
• I would believe this would work with learnr v0.9.2 and the next release of v0.10.0
• Existing users should not have to alter their code to get the expected behavior

cc @matthew-brett @reuning

[codecov[bot]]

Codecov Report

Merging #233 into master will increase coverage by <.01%. The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
+ Coverage   92.11%   92.11%   +<.01%     
==========================================
  Files          47       47              
  Lines        2586     2588       +2     
==========================================
+ Hits         2382     2384       +2     
  Misses        204      204

Impacted Files	Coverage Δ
R/utils-learnr.R	0% <0%> (ø)	:arrow_up:
R/test-exercise.R	88.37% <100%> (+0.56%)	:arrow_up:

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