remove test for base R error in execution tests

[simonpcouch]

Not sure that this was the original intent of the test:

parsnip/tests/testthat/_snaps/boost_tree_xgboost.md

Lines 3 to 7 in 4562458

	Code
	res <- parsnip::fit(hpc_xgboost, class ~ novar, data = hpc, control = ctrl)
	Condition
	Error:
	! object 'novar' not found

Noticed this in the transition to snapshot testing in #1190.

[simonpcouch]

There are actually quite a bit of these:

parsnip/tests/testthat/_snaps/mlp_keras.md

Lines 1 to 7 in 4562458

	# keras execution, classification
	Code
	res <- parsnip::fit(hpc_keras, class ~ novar, data = hpc, control = ctrl)
	Condition
	Error:
	! object 'novar' not found

parsnip/tests/testthat/_snaps/mlp_nnet.md

Lines 1 to 7 in 4562458

	# nnet execution, classification
	Code
	res <- parsnip::fit(hpc_nnet, class ~ novar, data = hpc, control = ctrl)
	Condition
	Error:
	! object 'novar' not found

Let's do all of these in one swoop if we do remove. :)

[simonpcouch]

Another:

parsnip/tests/testthat/_snaps/nearest_neighbor_kknn.md

Lines 11 to 15 in 4562458

	Code
	fit(hpc_basic, hpc_bad_form, data = hpc, control = ctrl)
	Condition
	Error:
	! object 'term' not found