Allow "special values" passed as `n` in `dbFetch()`

[nbenn]

As per DBI specification a backend may allow for "special values" to be passed as n in dbFetch():

n maximum number of records to retrieve per fetch. Use n = -1 or n = Inf to retrieve all pending records. Some implementations may recognize other special values.

Such a "special value" might for example be NA. Yet, there are tests that disallow this:

DBItest/R/spec-result-fetch.R

Lines 54 to 64 in 2c29217

	fetch_n_bad = function(con) {
	#' If the `n` argument is not an atomic whole number
	#' greater or equal to -1 or Inf, an error is raised,
	query <- trivial_query()
	res <- local_result(dbSendQuery(con, query))
	expect_error(dbFetch(res, -2))
	expect_error(dbFetch(res, 1.5))
	expect_error(dbFetch(res, integer()))
	expect_error(dbFetch(res, 1:3))
	expect_error(dbFetch(res, NA_integer_))
	},

Generally, such tests should probably be removed. If the specification is narrowed down a bit (to scalar values for examples), some tests may remain.

[krlmlr]

Agree. We can skip them for now, with a reference to this issue.

[github-actions]

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