patch 8.0.0861: still many old style tests

Problem:    Still many old style tests.
Solution:   Convert several tests to new style. (Yegappan Lakshmanan)
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
new file mode 100644
index 0000000..24c6ef5
--- /dev/null
+++ b/src/testdir/test_let.vim
@@ -0,0 +1,27 @@
+" Tests for the :let command.
+
+func Test_let()
+  " Test to not autoload when assigning.  It causes internal error.
+  set runtimepath+=./sautest
+  let Test104#numvar = function('tr')
+  call assert_equal("function('tr')", string(Test104#numvar))
+
+  let a = 1
+  let b = 2
+
+  let out = execute('let a b')
+  let s = "\na                     #1\nb                     #2"
+  call assert_equal(s, out)
+
+  let out = execute('let {0 == 1 ? "a" : "b"}')
+  let s = "\nb                     #2"
+  call assert_equal(s, out)
+
+  let out = execute('let {0 == 1 ? "a" : "b"} a')
+  let s = "\nb                     #2\na                     #1"
+  call assert_equal(s, out)
+
+  let out = execute('let a {0 == 1 ? "a" : "b"}')
+  let s = "\na                     #1\nb                     #2"
+  call assert_equal(s, out)
+endfunc