remove class before testing if validate output is false

A weird bug just popped up and I don't know where it's coming from, but
effectively, when a value with a class of "error" is passed to
`encodeString()`, it fails with:

Error in UseMethod("conditionMessage") :
  no applicable method for 'conditionMessage' applied to an object of class "error"

This never used to happen, so I'm fixing it here.

tests/testthat/test-validate_config.R

@@ -41,7 +41,7 @@ test_that("Missing files returns an invalid config with an immediate message", {
   suppressMessages({
     expect_message(out <- validate_config(hub_path = tmp), "File does not exist")
   })
-  expect_false(out)
+  expect_false(unclass(out))
 })
 test_that("Config for samples fail correctly", {
   skip_if_offline()

tests/testthat/test-validate_model_metadata_schema.R

@@ -6,7 +6,7 @@ test_that("Missing files returns an invalid config with an immediate message", {
       "File does not exist"
     )
   })
-  expect_false(out)
+  expect_false(unclass(out))
 })
 
 test_that("validate_model_metadata_schema works", {
@@ -28,7 +28,7 @@ test_that("validate_model_metadata_schema works", {
       )
     )
   )
-  expect_false(out_error)
+  expect_false(unclass(out_error))
   expect_snapshot(out_error) # prints .Last.value
   expect_snapshot(print(out_error)) # prints out_error
   expect_snapshot(str(attr(out_error, "errors")))
@@ -51,6 +51,6 @@ test_that("validate_model_metadata_schema errors for imparsable json", {
       "SyntaxError"
     )
   })
-  expect_false(out)
+  expect_false(unclass(out))
 
 })