patch 8.2.4930: interpolated string expression requires escaping

Problem:    Interpolated string expression requires escaping.
Solution:   Do not require escaping in the expression.
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 7e1a923..fb315e8 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -387,9 +387,8 @@
   let text = 'text'
   call assert_equal('text{{', $'{text .. "{{"}')
   call assert_equal('text{{', $"{text .. '{{'}")
-  " FIXME: should not need to escape quotes in the expression
-  call assert_equal('text{{', $'{text .. ''{{''}')
-  call assert_equal('text{{', $"{text .. \"{{\"}")
+  call assert_equal('text{{', $'{text .. '{{'}')
+  call assert_equal('text{{', $"{text .. "{{"}")
 endfunc
 
 " Test for the setting a variable using the heredoc syntax.