The defaults are:
ProgressReporter for interactive, non-parallel; override with
testthat.default_reporter
ParallelProgressReporter for interactive, parallel packages;
override with testthat.default_parallel_reporter
CompactProgressReporter for single-file interactive; override with
testthat.default_compact_reporter
CheckReporter for R CMD check; override with testthat.default_check_reporter
default_reporter()
default_parallel_reporter()
default_compact_reporter()
check_reporter()