Add unit tests

Change-Id: Ic8ad5c8fc6a14d6220bc049400c440bb91732e9e
diff --git a/tests/testthat.R b/tests/testthat.R
new file mode 100644
index 0000000..992d351
--- /dev/null
+++ b/tests/testthat.R
@@ -0,0 +1,4 @@
+library("testthat")
+library("RKorAPClient")
+
+test_check("RKorAPClient")