ox-texinfo: Fix invalid syntax in Texinfo version detection code

* lisp/ox-texinfo.el (org-texinfo-supports-math-p): Fix the incorrect
syntax @displaymath{1 + 1 = 2} used to detect whether Texinfo supports
TeX "math mode".  Instead, use the correct syntax @math{1 + 1 = 2}.

lisp/ox-texinfo.el

@@ -32,6 +32,8 @@
 (require 'cl-lib)
 (require 'ox)
 
+(eval-when-compile (require 'subr-x))
+
 (defvar orgtbl-exp-regexp)
 (defvar org-texinfo-supports-math--cache)
 
@@ -2025,12 +2027,14 @@ Once computed, the results remain cached."
   (unless (boundp 'org-texinfo-supports-math--cache)
     (setq org-texinfo-supports-math--cache
           (let ((math-example "1 + 1 = 2"))
-            (let* ((input-file
-                    (make-temp-file "test" nil ".info"))
-                   (input-content
-                    (concat (format "@setfilename %s" input-file) "\n"
-                            "@node Top" "\n"
-                            (format "@displaymath{%s}" math-example) "\n")))
+            (let* ((input-file (make-temp-file "test" nil ".info"))
+                   (input-content (string-join
+                                   (list (format "@setfilename %s" input-file)
+                                         "@node Top"
+                                         "@displaymath"
+                                         math-example
+                                         "@end displaymath")
+                                   "\n")))
               (with-temp-file input-file
                 (insert input-content))
               (let* ((output-file (org-texinfo-compile input-file))

testing/lisp/test-ox-texinfo.el

@@ -24,6 +24,8 @@
 (require 'cl-lib)
 (require 'ox-texinfo)
 
+(eval-when-compile (require 'subr-x))
+
 (unless (featurep 'ox-texinfo)
   (signal 'missing-test-dependency "org-export-texinfo"))
 
@@ -292,5 +294,35 @@
              nil
              '(:with-latex t))))))
 
+
+;;; End-to-end
+
+(ert-deftest test-ox-texinfo/end-to-end-inline ()
+  "Test end-to-end with inline TeX fragment."
+  (should
+   (org-test-with-temp-text
+    "$a^2 = b$"
+    (let ((export-buffer "*Test Texinfo Export*")
+          (org-export-show-temporary-export-buffer nil))
+      (org-export-to-buffer 'texinfo export-buffer
+        nil nil nil nil nil
+        #'texinfo-mode)))))
+
+(ert-deftest test-ox-texinfo/end-to-end-sanity-check-displayed ()
+  "Test end-to-end with LaTeX environment."
+  (should
+   (org-test-with-temp-text
+    (string-join
+     (list "\\begin{equation}"
+           "a ^ 2 = b"
+           "b ^ 2 = c"
+           "\\end{equation}")
+     "\n")
+    (let ((export-buffer "*Test Texinfo Export*")
+          (org-export-show-temporary-export-buffer nil))
+      (org-export-to-buffer 'texinfo export-buffer
+        nil nil nil nil nil
+        #'texinfo-mode)))))
+
 (provide 'test-ox-texinfo)
 ;;; test-ox-texinfo.el end here