release: Bump required version for testthat

[schloerke]

testthat 3.3.0 introduced two bugs fixed in r-lib/testthat#2294

Bug: r-lib/testthat#2288

Error "no 'restart' 'continue_test' found" when using shinytest2::AppDriver$expect_values() within test_that()

Bug: r-lib/testthat#2293

shinytest2::AppDriver$expect_values() fails on CI with invalid argument type during snapshot

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Once testthat is released, we should bump the testthat min package version, drop the DESCRIPTION Remotes section, and release shinytest2. (PR: #425 )

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Workaround:

Install {shinytest2} from GitHub (which will install {testthat} from GitHub):

pak::pak("rstudio/shinytest2")