Establish first unit tests

[DavidBard]

For automated testing on Travis (#2) and AppVeyor (#3). Adapt from https://github.com/OuhscBbmc/referral-exposure/tree/master/tests.

• Create ./tests/test-all.R. Make sure there's an empty last line (not even a space or tab).

#Modeled after the R6 testing structure: https://github.com/wch/R6/blob/master/tests/testthat.R
library(testthat)
library(ReferralExposure)

testthat::test_check("ReferralExposure")

• Create .tests/testhat/rest-basic.R. Make sure there's an empty last line (not even a space or tab).

library(testthat)
context("Basic Functions")

test_that("smoke-test", {
returned <- basic(3)
expect_true(!is.null(returned))
})

test_that("vector-test", {
expected <- 4:7
returned <- basic(3:6)
expect_equal(returned, expected)
})

• Verify that this fails your local check (because we haven't yet created the function basic()).