gdb/testsuite: fix directory name in test name

In the commit:

  commit 4793f551a5aa68522fd5fbbb7e8f621148f410cd
  Date:   Mon Nov 27 13:33:17 2023 +0000

      gdb: allow use of ~ in 'save gdb-index' command

I added a test which has a directory name within the GDB command,
which then appears in the test name as I failed to give the test a
better name.

Fixed in this commit.