[gdb/testsuite] Make pass message unique in gdb-index.exp for cc-with-dwz-m

[gdb/testsuite] Make pass message unique in gdb-index.exp for cc-with-dwz-m

With cc-with-dwz-m, we get:
...
PASS: gdb.dwarf2/gdb-index.exp: objcopy
PASS: gdb.dwarf2/gdb-index.exp: objcopy
...

Make the pass message unique by using with_test_prefix:
...
PASS: gdb.dwarf2/gdb-index.exp: objcopy
PASS: gdb.dwarf2/gdb-index.exp: modify dwz file: objcopy
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-09-29  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/gdb-index.exp: Use with_test_prefix for second objcopy.