patch 8.2.4883: string interpolation only works in heredoc

Problem:    String interpolation only works in heredoc.
Solution:   Support interpolated strings.  Use syntax for heredoc consistent
            with strings, similar to C#. (closes #10327)
diff --git a/src/testdir/test_debugger.vim b/src/testdir/test_debugger.vim
index 5aa7bab..17d3616 100644
--- a/src/testdir/test_debugger.vim
+++ b/src/testdir/test_debugger.vim
@@ -377,7 +377,7 @@
   let expected =<< eval trim END
     Oldval = "10"
     Newval = "11"
-    `=fnamemodify('Xtest.vim', ':p')`
+    {fnamemodify('Xtest.vim', ':p')}
     line 1: let g:Xtest_var += 1
   END
   call RunDbgCmd(buf, ':source %', expected)
@@ -385,7 +385,7 @@
   let expected =<< eval trim END
     Oldval = "11"
     Newval = "12"
-    `=fnamemodify('Xtest.vim', ':p')`
+    {fnamemodify('Xtest.vim', ':p')}
     line 1: let g:Xtest_var += 1
   END
   call RunDbgCmd(buf, ':source %', expected)