tests/functional2/lang: Migrate eof tests

Call it a bug, call it a feature, but the old testing framework
ungracefully shits itself when it has no tests to run … in other words,
we finally migrated all the parse-fail tests 🎉

Change-Id: Ibd7f1c04d9a396a20f14361af7924f0074d1ac23

tests/functional2/lang/parser-eof/parse-fail-string.err.exp

@@ -0,0 +1,5 @@
+error: syntax error, unexpected end of file, expecting '"'
+       at /pwd/in.nix:3:6:
+            2| # Note that this file must not end with a newline.
+            3| a 1"$
+             |      ^