gdb/testsuite: Remove duplicates from gdb.base/define-prefix.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/define-prefix.exp ...
    DUPLICATE: gdb.base/define-prefix.exp: define user command: ghi-prefix-cmd

Fix by adjusting test names.

Tested on x86_64-linux.
diff --git a/gdb/testsuite/gdb.base/define-prefix.exp b/gdb/testsuite/gdb.base/define-prefix.exp
index 18e5651..47e0a17 100644
--- a/gdb/testsuite/gdb.base/define-prefix.exp
+++ b/gdb/testsuite/gdb.base/define-prefix.exp
@@ -58,7 +58,7 @@
     "define user command: abc-prefix def-prefix ghi-prefix-cmd" {
 	-re "Type commands for definition of \"abc-prefix def-prefix ghi-prefix-cmd\".\r\nEnd with a line saying just \"end\".\r\n>$" {
 	    gdb_test "end" "" \
-		"define user command: ghi-prefix-cmd"
+		"define empty user command: ghi-prefix-cmd"
 	}
     }
 gdb_test_no_output "define-prefix abc-prefix def-prefix ghi-prefix-cmd"