Provide way to determine if execution is via `testthat::test_*()`

[jennybc]

Can testthat provide a way to determine if execution is inside testthat::test_*()? This would be useful when running tests semi-interactively, such as test(filter = "foo") in the Console:

• You want to disable a progress reporter (Do not show spinner when running tests interactively tidyverse/reprex#111).
• You want to NOT ask the user for permission to do something.