Move code to Internal module and add tests

test/HCatSpec.hs

@@ -0,0 +1,15 @@
+{-# LANGUAGE GHC2021 #-}
+
+module HCatSpec (spec) where
+
+import HCat.Internal (parseArgs)
+
+import Test.Hspec
+
+spec :: Spec
+spec = do
+  describe "parseArgs" do
+    it "returns the filename if given a single argument" do
+      parseArgs ["foo"] `shouldBe` Right "foo"
+      parseArgs [] `shouldBe` Left "No filename given!"
+      parseArgs ["foo", "bar"] `shouldBe` Left "Only a single file is supported"