Helpers to set up test connections

[hadley]

For local testing, we can lean on SQLite a lot more, and for macs at least, automatically look in some common brew paths to reduce the amount of setup needed.

[hadley]

Start for SQLite:

sqlite_driver <- function() {
  # Assume that if SQLite driver is setup up it's correct
  drivers <- unique(odbcListDrivers()$name)
  if ("SQLite" %in% drivers) {
    return("SQLite")
  }
  
  if (Sys.info()["sysname"] == "Darwin") {
    if (Sys.which("brew") == "") {
      abort("Homebrew is not installed")
    }
    
    paths <- system("brew list sqliteodbc", intern = TRUE)
    if (attr(paths, "status") != 0) {
      abort("Failed to list sqliteodbc brew file")
    }
    paths[basename(paths) == "libsqlite3odbc.dylib"]
  } else {
    abort("Autodiscovery currently only supported on macOS")
  }
}

Maybe this becomes sqlite_default_driver() and is used in #659?

[hadley]

And need to pull out non-DBItest tests into their own test_that() blocks with helper for getting a connection. Should come before DBItests, and maybe gate DBItests behind an env var it's easier to iterate locally, without running every single DBItest (which is only needed occasionally and on CI)