Generate a defmacro form that transforms code like this:
(is (string= result (dummy-strcat (subseq result 0 split-point) (subseq result split-point))))

Pattern to recognize: Macro call that should be transformed by is

Macro definition:
(defmacro is (test &rest reason-args)
  "The DWIM checking operator.

If TEST returns a true value a test-passed result is generated,
otherwise a test-failure result is generated. The reason, unless
REASON-ARGS is provided, is generated based on the form of TEST:

 (predicate expected actual) - Means that we want to check
 whether, according to PREDICATE, the ACTUAL value is
 in fact what we EXPECTED.

 (predicate value) - Means that we want to ensure that VALUE
 satisfies PREDICATE.

 Wrapping the TEST form in a NOT simply produces a negated reason
 string."
  (assert (listp test)
          (test)
          "Argument to IS must be a list, not ~S" test)
  (let (bindings effective-test default-reason-args)
    (with-gensyms (e a v)
      (flet ((process-entry (predicate expected actual &optional negatedp)
               ;; make sure EXPECTED is holding the entry that starts with 'values
               (when (and (consp actual)
                          (eq (car actual) 'values))
                 (assert (not (and (consp expected)
                                   (eq (car expected) 'values))) ()
                                   "Both the expected and actual part is a values expression.")
                 (rotatef expected actual))
               (let ((setf-forms))
                 (if (and (consp expected)
                          (eq (car expected) 'values))
                     (progn
                       (setf expected (copy-list expected))
                       (setf setf-forms (loop for cell = (rest expected) then (cdr cell)
                                              for i from 0
                                              while cell
                                              when (eq (car cell) '*)
                                              collect `(setf (elt ,a ,i) nil)
                                              and do (setf (car cell) nil)))
                       (setf bindings (list (list e `(list ,@(rest expected)))
                                            (list a `(multiple-value-list ,actual)))))
                     (setf bindings (list (list e expected)
                                          (list a actual))))
                 (setf effective-test `(progn
                                         ,@setf-forms
                                         ,(if negatedp
                                              `(not (,predicate ,e ,a))
                                              `(,predicate ,e ,a)))))))
        (list-match-case test
          ((not (?predicate ?expected ?actual))
           (process-entry ?predicate ?expected ?actual t)
           (setf default-reason-args
                 (list "~2&~S~2% evaluated to ~2&~S~2% which is ~2&~S~2%to ~2&~S~2% (it should not be)"
                       `',?actual a `',?predicate e)))
          ((not (?satisfies ?value))
           (setf bindings (list (list v ?value))
                 effective-test `(not (,?satisfies ,v))
                 default-reason-args
                 (list "~2&~S~2% evaluated to ~2&~S~2% which satisfies ~2&~S~2% (it should not)"
                       `',?value v `',?satisfies)))
          ((?predicate ?expected ?actual)
           (process-entry ?predicate ?expected ?actual)
           (setf default-reason-args
                 (list "~2&~S~2% evaluated to ~2&~S~2% which is not ~2&~S~2% to ~2&~S~2%"
                       `',?actual a `',?predicate e)))
          ((?satisfies ?value)
           (setf bindings (list (list v ?value))
                 effective-test `(,?satisfies ,v)
                 default-reason-args
                 (list "~2&~S~2% evaluated to ~2&~S~2% which does not satisfy ~2&~S~2%"
                       `',?value v `',?satisfies)))
          (?_
           (setf bindings '()
                 effective-test test
                 default-reason-args (list "~2&~S~2% was NIL." `',test)))))
      `(let ,bindings
         (if ,effective-test
             (add-result 'test-passed :test-expr ',test)
             (process-failure ',test
                              ,@(or reason-args default-reason-args)))))))

Macroexpansion:
(let ((#:e167 result) (#:a168 (dummy-strcat (subseq result 0 split-point) (subseq result split-point)))) (if (progn (string= #:e167 #:a168)) (add-result (quote test-passed) :test-expr (quote (string= result (dummy-strcat (subseq result 0 split-point) (subseq result split-point))))) (process-failure (quote (string= result (dummy-strcat (subseq result 0 split-point) (subseq result split-point)))) "~2&~S~2% evaluated to ~2&~S~2% which is not ~2&~S~2% to ~2&~S~2%" (quote (dummy-strcat (subseq result 0 split-point) (subseq result split-point))) #:a168 (quote string=) #:e167)))