Remove `context` from `testthat` Tests

[njlyon0]

According to the chapter on testing in R Packages 2e, the context function is superseded in testthat version 3.0 and can be removed (context is provided by test file name and location in package). Not sure we'll get a flag from CRAN if we don't remove it but seems worthwhile to change while we're prepping for a new version release to CRAN.