[R-package] switch to SummaryReporter, remove context(), other testthat changes

[jameslamb]

Contributes to #223

Starts some cleanup of the project's {testthat} code, adapting to things that have changed in that library of the last few years.

• removes context()... unnecessary now that {testthat} infers it from the test file name
  • ref: [R-package] [tests] remove uses of testthat::context() microsoft/LightGBM#4915
• stops unsetting R_TESTS
  • the reasons behind system quoting fails with testthat + CMD check r-lib/testthat#144 no longer apply here
• removes rm(list = ls()) at the end of every test
  • *these are not necessary any more, and could be a source of issues in the future if they interact in bad ways with the test collection and loading mechanisms used by {testthat}
• switches to testthat::SummaryReporter, to get nice output like this: