signal buttercup-dynamic-binding-error when lexical-binding != t

The describe macro will signal buttercup-dynamic-binding-error if it
is expanded when lexical-binding it not t.
This should prevent test files without `lexical-binding: t' from being
loaded.
buttercup-run-discover will report the file error in the same way as
other errors encountered while loading files.

buttercup.el

@@ -58,6 +58,9 @@
 ;; inconsistencies.
 (define-error 'buttercup-internals-error "Internal buttercup error"
               'buttercup-error-base)
+;; Raised when expanding `describe` macros whithout lexical-binding: t
+(define-error 'buttercup-dynamic-binding-error "Lexical binding is not enabled"
+              'buttercup-error-base)
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;; wrapper function manipulation
@@ -954,6 +957,9 @@ form.")
 DESCRIPTION is a string. BODY is a sequence of instructions,
 mainly calls to `describe', `it' and `before-each'."
   (declare (indent 1) (debug (&define sexp def-body)))
+  (unless lexical-binding
+    (signal 'buttercup-dynamic-binding-error
+            "buttercup requires `lexical-binding' to be t"))
   (let ((new-body
          (cond
           ((eq (elt body 0) :var)