patch 8.1.2187: error for bad regexp even though regexp is not used

Problem:    Error for bad regexp even though regexp is not used when writing
            a file. (Arseny Nasokin)
Solution:   Ignore regexp errors. (closes #5059)
diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index 4b0a6d6..8eb2e78 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -1,4 +1,4 @@
-" Tests for the writefile() function.
+" Tests for the writefile() function and some :write commands.
 
 func Test_writefile()
   let f = tempname()
@@ -16,6 +16,11 @@
   call delete(f)
 endfunc
 
+func Test_writefile_ignore_regexp_error()
+  write Xt[z-a]est.txt
+  call delete('Xt[z-a]est.txt')
+endfunc
+
 func Test_writefile_fails_gently()
   call assert_fails('call writefile(["test"], "Xfile", [])', 'E730:')
   call assert_false(filereadable("Xfile"))