patch 8.1.1585: :let-heredoc does not trim enough

Problem:    :let-heredoc does not trim enough.
Solution:   Trim indent from the contents based on the indent of the first
            line.  Use let-heredoc in more tests.
diff --git a/src/testdir/test_xxd.vim b/src/testdir/test_xxd.vim
index cf47c8a..93cfedb 100644
--- a/src/testdir/test_xxd.vim
+++ b/src/testdir/test_xxd.vim
@@ -96,10 +96,10 @@
   exe '0r! ' . s:xxd_cmd . ' -i XXDfile'
   $d
   let expected =<< trim [CODE]
-  unsigned char XXDfile[] = {
-    0x54, 0x45, 0x53, 0x54, 0x61, 0x62, 0x63, 0x64, 0x30, 0x39, 0x0a
-  };
-  unsigned int XXDfile_len = 11;
+    unsigned char XXDfile[] = {
+      0x54, 0x45, 0x53, 0x54, 0x61, 0x62, 0x63, 0x64, 0x30, 0x39, 0x0a
+    };
+    unsigned int XXDfile_len = 11;
   [CODE]
 
   call assert_equal(expected, getline(1,'$'), s:Mess(s:test))
@@ -112,10 +112,10 @@
     exe '0r! ' . s:xxd_cmd . ' -i ' . arg . ' XXDfile'
     $d
     let expected =<< trim [CODE]
-    unsigned char XXDFILE[] = {
-      0x54, 0x45, 0x53, 0x54, 0x61, 0x62, 0x63, 0x64, 0x30, 0x39, 0x0a
-    };
-    unsigned int XXDFILE_LEN = 11;
+      unsigned char XXDFILE[] = {
+        0x54, 0x45, 0x53, 0x54, 0x61, 0x62, 0x63, 0x64, 0x30, 0x39, 0x0a
+      };
+      unsigned int XXDFILE_LEN = 11;
     [CODE]
     call assert_equal(expected, getline(1,'$'), s:Mess(s:test))
   endfor