patch 8.2.5165: import test fails because 'diffexpr' isn't reset

Problem:    Import test fails because 'diffexpr' isn't reset.
Solution:   Reset 'diffexpr'.

src/testdir/test_vim9_import.vim

@@ -1234,6 +1234,8 @@ def Run_Test_import_in_diffexpr()
   redraw
 
   diffoff!
+  set diffexpr=
+  set diffopt&
   bwipe!
   bwipe!
   delete('Xdiffexpr')

src/version.c

@@ -735,6 +735,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5165,
 /**/
     5164,
 /**/