test-ob-scheme.el: Disable testing for older Emacs unsupported by Geiser

* testing/lisp/test-ob-scheme.el (version<=): Throw
'missing-test-dependency unless minimum Emacs version required by
Geiser is used.

Link: https://orgmode.org/list/87r0og2dz7.fsf@mail.jao.io

testing/lisp/test-ob-scheme.el

@@ -27,6 +27,8 @@
   (signal 'missing-test-dependency "Support for Scheme code blocks"))
 (unless (featurep 'geiser)
   (signal 'missing-test-dependency "geiser"))
+(unless (version<= "27.1" emacs-version)
+  (signal 'missing-test-dependency "Geiser required for Scheme code blocks needs Emacs >=27.1"))
 
 (ert-deftest test-ob-scheme/tables ()
   "Test table output."