Reorganize tests and benchmark scripts.

Mainly to get rid of one top level directory. But this will
also be useful when there are tests of the embedding API.

test/language/module/unknown_variable/module.wren

@@ -0,0 +1,3 @@
+// nontest
+var Module = "from module"
+IO.print("ran module")

test/language/module/unknown_variable/unknown_variable.wren

@@ -0,0 +1,3 @@
+// Should execute the module:
+// expect: ran module
+import "module" for DoesNotExist // expect runtime error: Could not find a variable named 'DoesNotExist' in module 'module'.