tests: write_log gains unit tests (#58)

Seems like this function didn't have any tests to begin with!

Closes #57

tests/testthat/test-write_log.R

@@ -0,0 +1,13 @@
+test_that("writes to log_path as expected", {
+  msg <- withr::with_tempdir({
+    log_path <- getwd()
+    write_log("Hello World!", log_path)
+    readLines(file.path(log_path, "error_messages.txt"))
+  })
+
+  expect_match(msg, "Hello World!")
+})
+
+test_that("multiplication works", {
+  expect_warning(warn <- write_log("Hello World!"))
+})