unknown test_file() warning from testthat package "No deferred expressions to run"

Question

I'm getting the following output when running a test file through the test_file() function, but not when executing the test file directly:

[ FAIL 0 | WARN 0 | SKIP 0 | PASS 33 ]No deferred expressions to run

What is the "No deferred expressions to run" warning? I do not get this on an older version of the testthat library.