patch 9.1.0091: Syntax test fails when run with non C locale

Problem:  Syntax test fails when run with non C locale
Solution: Run syntax tests with C locale, clean up Xtestscript file,
          strip environment variables from GetVimCommand()
          (h-east)

closes: #14007

Co-authored-by: h-east <h.east.727@gmail.com>
Signed-off-by: h-east <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/testdir/runtest.vim b/runtime/syntax/testdir/runtest.vim
index 80f1eff..d2ce3d1 100644
--- a/runtime/syntax/testdir/runtest.vim
+++ b/runtime/syntax/testdir/runtest.vim
@@ -237,11 +237,12 @@
 call Message('skipped: ' .. skipped_count)
 call AppendMessages('== SUMMARY ==')
 
-" Matching "if 1" at the start.
-endif
-
 if len(failed_tests) > 0
   " have make report an error
   cquit
 endif
+
+" Matching "if 1" at the start.
+endif
+
 qall!