patch 8.2.4611: typos in tests; one lua line not covered by test

Problem:    Typos in tests; one lua line not covered by test.
Solution:   Fix typos. Add test case. (Dominique Pellé, closes #9994)
diff --git a/src/testdir/test_regexp_latin.vim b/src/testdir/test_regexp_latin.vim
index 166488b..71915be 100644
--- a/src/testdir/test_regexp_latin.vim
+++ b/src/testdir/test_regexp_latin.vim
@@ -799,7 +799,7 @@
   bwipe!
 endfunc
 
-" Check a pattern with a look beind crossing a line boundary
+" Check a pattern with a look behind crossing a line boundary
 func Test_lookbehind_across_line()
   new
   call append(0, ['Behind:', 'asdfasd<yyy', 'xxstart1', 'asdfasd<yy',