Add a spec for Source.

semantic-diff.cabal

@@ -116,6 +116,7 @@ test-suite semantic-diff-test
                      , InterpreterSpec
                      , PatchOutputSpec
                      , RangeSpec
+                     , Source.Spec
                      , TermSpec
   build-depends:       base
                      , bifunctors

test/Source/Spec.hs

@@ -0,0 +1,9 @@
+module Source.Spec where
+
+import Prologue
+import Source
+import Test.Hspec
+
+spec :: Spec
+spec = do
+  pure ()