Properly report errors about unbound ERT test symbols.

Assertions should only be used to check internal consistency within a
package, not to check arguments passed by callers.  Instead, define
and use a new error symbol.

* lisp/emacs-lisp/ert.el (ert-test-unbound): New error symbol.
(ert-select-tests): Use it.

* test/lisp/emacs-lisp/ert-tests.el (ert-test-select-undefined): New
unit test.

* etc/NEWS: Document new behavior.

etc/NEWS

@@ -175,6 +175,9 @@ When environment variable 'EMACS_TEST_JUNIT_REPORT' is set, ERT
 generates a JUnit test report under this file name.  This is useful
 for Emacs integration into CI/CD test environments.
 
+*** Unbound test symbols now signal an 'ert-test-unbound' error.
+This affects the 'ert-select-tests' function and its callers.
+
 ** Emoji
 
 +++

lisp/emacs-lisp/ert.el

@@ -1012,7 +1012,8 @@ contained in UNIVERSE."
                           universe))))
     ((pred ert-test-p) (list selector))
     ((pred symbolp)
-     (cl-assert (ert-test-boundp selector))
+     (unless (ert-test-boundp selector)
+       (signal 'ert-test-unbound (list selector)))
      (list (ert-get-test selector)))
     (`(,operator . ,operands)
      (cl-ecase operator
@@ -1020,7 +1021,9 @@ contained in UNIVERSE."
         (mapcar (lambda (purported-test)
                   (pcase-exhaustive purported-test
                     ((pred symbolp)
-                     (cl-assert (ert-test-boundp purported-test))
+                     (unless (ert-test-boundp purported-test)
+                       (signal 'ert-test-unbound
+                               (list purported-test)))
                      (ert-get-test purported-test))
                     ((pred ert-test-p) purported-test)))
                 operands))
@@ -1059,6 +1062,8 @@ contained in UNIVERSE."
         (cl-remove-if-not (car operands)
                           (ert-select-tests 't universe)))))))
 
+(define-error 'ert-test-unbound "ERT test is unbound")
+
 (defun ert--insert-human-readable-selector (selector)
   "Insert a human-readable presentation of SELECTOR into the current buffer."
   ;; This is needed to avoid printing the (huge) contents of the

test/lisp/emacs-lisp/ert-tests.el

@@ -495,6 +495,12 @@ This macro is used to test if macroexpansion in `should' works."
     (should (equal (ert-select-tests '(tag b) (list test)) (list test)))
     (should (equal (ert-select-tests '(tag c) (list test)) '()))))
 
+(ert-deftest ert-test-select-undefined ()
+  (let* ((symbol (make-symbol "ert-not-a-test"))
+         (data (should-error (ert-select-tests symbol t)
+                             :type 'ert-test-unbound)))
+    (should (eq (cadr data) symbol))))
+
 
 ;;; Tests for utility functions.
 (ert-deftest ert-test-parse-keys-and-body ()