tests: avoid indenting with tabs

tabs indentation is up to Emacs' indent functions, we don't need to
test it. Avoiding tabs in tests simplifies comparing the current
indentation and allows to increase test coverage in the next commit.

test/tla-pcal-mode-test.el

@@ -73,10 +73,11 @@
         (insert line "\n"))
       (goto-char (point-min))
       (forward-line n) ; Add something in to place point at various places in line?
-      (let ((expected-indent (current-indentation)))
+      (let ((expected-indent (current-indentation))
+            (pcal-mode-indent-offset 2)
+            (indent-tabs-mode nil))
         (delete-horizontal-space)
-        (let* ((pcal-mode-indent-offset 2) ; dynamic binding because of defvar
-               (actual-indent (funcall indent-func))
+        (let* ((actual-indent (funcall indent-func))
                (exp-line (elt lines n)))
           ;; Add enough extra data to make clear where the failure happened
           (should (equal (list expected-indent (list 'line n) exp-line)