[gdb/testsuite] Fix gdb.base/gdb-index-err.exp for root user

[gdb/testsuite] Fix gdb.base/gdb-index-err.exp for root user

When running test-case gdb.base/gdb-index-err.exp in a container as root user,
I run into:
...
FAIL: gdb.base/gdb-index-err.exp: flag=: \
  try to write index to a non-writable directory
FAIL: gdb.base/gdb-index-err.exp: flag=-dwarf-5: \
  try to write index to a non-writable directory
...

The test-case creates a directory without write permissions:
...
$ ls -ald private
dr-xr-xr-x 2 root root 4096 Dec 29 06:26 private/
...
but apparently the root user is still able to write in it.

Fix this by making the test unsupported for the root user.

Tested on x86_64-linux.

Reviewed-By: Lancelot SIX <lancelot.six@amd.com>
PR testsuite/31197
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31197