Added example snippets of the presentation.

example/example.R

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+# Example function to be tested.
+linear_transformation <- function(x) {
+    return (2*x + 1)
+}

example/test/test_linear_transformation.R

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+source("../example.R", chdir=TRUE)
+library(testthat)
+
+test_that("Real numbers are transformed to odd numbers.", {
+  expect_equal(linear_transformation(3) %% 2, 1)
+  expect_equal(linear_transformation(4) %% 2, 1)
+})
+
+test_that("Positive numbers remain positive.", {
+  expect_true(linear_transformation(3) > 0)
+  expect_true(linear_transformation(10) > 0)
+})