update testthat version (#359)

We need to bump up `testthat` version because we're using `testthat::it`
function in unit tests.

This function was introduced in `3.1.8`:
https://github.com/r-lib/testthat/blob/v3.1.8/R/describe.R#L89

This update will fix the `Dependency Test - min isolated`:

Author: donyunardi

DESCRIPTION

@@ -40,7 +40,7 @@ Imports:
 Suggests:
     knitr (>= 1.42),
     rmarkdown (>= 2.23),
-    testthat (>= 3.1.5),
+    testthat (>= 3.2.2),
     withr (>= 2.0.0)
 VignetteBuilder:
     knitr,

tests/testthat/test-cdisc_join_keys.R

@@ -1,5 +1,5 @@
 testthat::test_that("default_cdisc_join_keys is assigned in package environment", {
-  testthat::expect_true(exists("default_cdisc_join_keys"))
+  testthat::expect_true(exists("default_cdisc_join_keys", where = asNamespace("teal.data")))
   testthat::expect_gt(length(default_cdisc_join_keys), 0)
 })