Add a test for type correctness of antiquotes

Antiquotes should evaluate to strings or paths.  This is usually
checked, except in the case where the antiquote makes up the entire
string, as in "${expr}".  This is optimised to expr, which discards
the runtime type checks / coercions.

tests/lang/eval-fail-bad-antiquote-1.nix

@@ -0,0 +1 @@
+"${x: x}"

tests/lang/eval-fail-bad-antiquote-2.nix

@@ -0,0 +1 @@
+"${./fnord}"

tests/lang/eval-fail-bad-antiquote-3.nix

@@ -0,0 +1 @@
+''${x: x}''