patch 9.1.0562: tests: inconsistency in test_findfile.vim

Problem:  tests: inconsistency in test_findfile.vim, it saves and
          restores 'shellslash', but doesn't actually set it
Solution: Set shellslash explicitly (zeertzjq)

closes: #15217

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_findfile.vim b/src/testdir/test_findfile.vim
index 20d5096..311020e 100644
--- a/src/testdir/test_findfile.vim
+++ b/src/testdir/test_findfile.vim
@@ -133,6 +133,7 @@
   let save_shellslash = &shellslash
   let save_dir = getcwd()
   set path=,,
+  set shellslash
   call CreateFiles()
   cd Xfinddir1