patch 8.2.0509: various code is not properly tested.

Problem:    various code is not properly tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5871)
diff --git a/src/testdir/test_expand.vim b/src/testdir/test_expand.vim
index d86fea4..383921b 100644
--- a/src/testdir/test_expand.vim
+++ b/src/testdir/test_expand.vim
@@ -84,6 +84,15 @@
   let $FOO="blue\tsky"
   call setline(1, "$FOO")
   call assert_equal("grep pat blue\tsky", expandcmd('grep pat <cfile>'))
+
+  " Test for expression expansion `=
+  let $FOO= "blue"
+  call assert_equal("blue sky", expandcmd("`=$FOO .. ' sky'`"))
+
+  " Test for env variable with spaces
+  let $FOO= "foo bar baz"
+  call assert_equal("e foo bar baz", expandcmd("e $FOO"))
+
   unlet $FOO
   close!
 endfunc