patch 8.2.0418: code in eval.c not sufficiently covered by tests

Problem:    Code in eval.c not sufficiently covered by tests.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5815)
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 85edddf..5882f53 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -420,6 +420,7 @@
 
   " Test expand(`=...`) i.e. backticks expression expansion
   call assert_equal('5', expand('`=2+3`'))
+  call assert_equal('3.14', expand('`=3.14`'))
 
   " clean up
   bw!