Do not collect backtraces for failed specs

There is no reason to collect backtraces for expect form where the
matcher detects a mismatch.  It's all buttercup infrastructure at that
point.

buttercup.el

@@ -2107,8 +2107,9 @@ ARGS according to `debugger'."
                ;; args is (error (signal . data) ....) where the tail
                ;; may be empty
                (cl-destructuring-bind (signal-type . data) (cl-second args)
-                 (unless (eq signal-type 'buttercup-pending)
-                   (buttercup--backtrace))))))
+                 (cl-case signal-type
+                   ((buttercup-pending buttercup-failed))
+                   (otherwise (buttercup--backtrace)))))))
 
 (defalias 'buttercup--mark-stackframe #'ignore
   "Marker to find where the backtrace start.")
@@ -2137,7 +2138,6 @@ ARGS according to `debugger'."
            ;;      condition-case   -- from buttercup-with-converted-ert-signals
            ;;       progn           -- the same
            ;;        ACTUAL CODE
-           ;; TODO: matchers that do not match should not collect backtrace
            ;; TODO: What about an error in a matcher?
            ;; TODO: What about :to-throw?
            ;; TODO: What about signals in before and after blocks?

tests/test-buttercup.el

@@ -1969,22 +1969,32 @@ before it's processed by other functions."
 	(fmakunbound 'bc-bt-foo)
 	(fmakunbound 'bc-bt-bar)
 	(fmakunbound 'bc-bt-baz))
-  (it "should be printed for each failed spec"
-    (with-local-buttercup
-      :reporter #'backtrace-reporter
-      (describe "suite"
-        (it "expect 2" (expect (+ 1 2) :to-equal 2))
-        (it "expect nil" (expect nil)))
-      (buttercup-run :noerror))
-    (expect (buttercup-output) :to-match
-            (rx string-start
-                (= 2 (seq (= 40 ?=) "\n"
-                          "suite expect " (or "2" "nil") "\n"
-                          "\n"
-                          "Traceback (most recent call last):\n"
-                          (* (seq "  " (+ not-newline) "\n"))
-                          (or "FAILED" "error") ": " (+ not-newline) "\n\n"))
-                string-end)))
+  (it "should not be collected or printed for failed specs"
+    (let (test-suites)
+      (spy-on 'buttercup--backtrace :and-call-through)
+      (with-local-buttercup
+       :reporter #'backtrace-reporter
+       (describe "suite"
+         (it "expect 2" (expect (+ 1 2) :to-equal 2))
+         (it "expect nil" (expect nil)))
+       (buttercup-run :noerror)
+       (setq test-suites buttercup-suites))
+      (expect 'buttercup--backtrace :not :to-have-been-called)
+      ;; Checking both if buttercup--backtrace have been called and
+      ;; the failure-stack value might be overkill
+      (cl-loop for spec in (buttercup-suite-children (car test-suites))
+               if (buttercup-spec-failure-stack spec)
+               collect (buttercup-spec-failure-stack spec) into stacks
+               finally (expect stacks :to-be nil))
+      (expect (buttercup-output) :to-match
+              (rx-let ((failure-report (name)
+                          (seq (= 40 ?=) "\n"
+                               "suite expect " name "\n"
+                               "FAILED: " (+ not-newline) "\n\n")))
+                (rx string-start
+                    (failure-report "2")
+                    (failure-report "nil")
+                    string-end)))))
   (describe "with style"
     :var (test-suites long-string)
     ;; Set up tests to test