Make order of UITest *.py test file processing deterministic

...hoping that this can prevent issues like the one seen with
1b0eb76eb1 "tdf#137617: sc: Add UItest" followed
by d1232ee6b5 "uitest: reset formula syntax back
to Calc A1" (i.e., where the first commit's Gerrit Jenkins build had apparently
only succeeded because it happened to run the tests in one specific order,
different from the order in which later, failing builds ran it)

Change-Id: I4f1b5ada10d60e77b45a0edeb89069fe9376b63e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/105325
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

uitest/test_main.py

@@ -37,7 +37,7 @@ def usage():
 
 def find_test_files(dir_path):
     valid_files = []
-    for f in os.listdir(dir_path):
+    for f in sorted(os.listdir(dir_path)):
         file_path = os.path.join(dir_path, f)
 
         # don't go through the sub-directories