[gdb/testsuite] Fix duplicate in gdb.mi/mi-var-cp.exp

[gdb/testsuite] Fix duplicate in gdb.mi/mi-var-cp.exp

With test-case gdb.mi/mi-var-cp.exp I run into this duplicate:
...
PASS: gdb.mi/mi-var-cp.exp: run to mi-var-cp.cc:104 (set breakpoint)
PASS: gdb.mi/mi-var-cp.exp: create varobj for s
PASS: gdb.mi/mi-var-cp.exp: create varobj for s
DUPLICATE: gdb.mi/mi-var-cp.exp: create varobj for s
...

This is due to a duplicate test name here:
...
$ cat -n gdb/testsuite/gdb.mi/mi-var-cp.cc
  ...
   100  int reference_to_struct ()
   101  {
   102    /*: BEGIN: reference_to_struct :*/
   103    S s = {7, 8};
   104    S& r = s;
   105    /*:
   106      mi_create_varobj S s "create varobj for s"
   107      mi_create_varobj R r "create varobj for s"
...

Fix this by using "create varobj for r" instead.

Tested on x86_64-linux.