local_con() test fixture

[romainfrancois]

Instead of :

  con <- dbConnect(duckdb())
  on.exit(dbDisconnect(con, shutdown = TRUE))

we'd have :

con <- local_con()

or some other name, to follow the guidelines from https://testthat.r-lib.org/articles/test-fixtures.html

something like (borrowed from https://github.com/hannes/duckdb-rfuns).

local_con <- function() {
  con <- dbConnect(duckdb())
  withr::defer_parent(dbDisconnect(con, shutdown=TRUE))
  con
}