patch 8.2.0401: not enough test coverage for evalvars.c

Problem:    Not enough test coverage for evalvars.c.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5804)

src/testdir/test_diffmode.vim

@@ -1065,3 +1065,32 @@ func Test_diff_maintains_change_mark()
   bwipe!
   bwipe!
 endfunc
+
+" Test for 'patchexpr'
+func Test_patchexpr()
+  let g:patch_args = []
+  func TPatch()
+    call add(g:patch_args, readfile(v:fname_in))
+    call add(g:patch_args, readfile(v:fname_diff))
+    call writefile(['output file'], v:fname_out)
+  endfunc
+  set patchexpr=TPatch()
+
+  call writefile(['input file'], 'Xinput')
+  call writefile(['diff file'], 'Xdiff')
+  %bwipe!
+  edit Xinput
+  diffpatch Xdiff
+  call assert_equal('output file', getline(1))
+  call assert_equal('Xinput.new', bufname())
+  call assert_equal(2, winnr('$'))
+  call assert_true(&diff)
+
+  call delete('Xinput')
+  call delete('Xdiff')
+  set patchexpr&
+  delfunc TPatch
+  %bwipe!
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab