Encourage braced expression in `test_that()` calls

[IndrajeetPatil]

More details in r-lib/styler#1154

Not sure if we can accommodate this in any of the existing linters.

[AshesITR]

Maybe add to brace_linter()?
I can think of a few other functions that should take braced expressions. For example reactive() and render*() from shiny.

[MichaelChirico]

Haha. Funnily enough I just saw this in the {testthat} NEWS:

test_that() no longer warns about the absence of {} since it no longer seems to be necessary.

👀 r-lib/testthat#2193

Maybe we should revert the associated PR here since it's not yet on CRAN?