gdb/testsuite: handle failure to start process for later attach test

gdb/testsuite: handle failure to start process for later attach test

Commit:

  commit b23903836007d1acaf7f8c059ab000ee83fcebfa
  Date:   Tue Mar 21 13:01:26 2023 +0100

      gdb: linux-namespaces: enter user namespace when appropriate

added a new test gdb.base/user-namespace-attach.exp.  It has been
reported that this test will sometimes fail, like this:

  (gdb) attach 184732
  Attaching to process 184732
  warning: process 184732 is a zombie - the process has already terminated
  ptrace: Operation not permitted.
  (gdb) FAIL: gdb.base/user-namespace-attach.exp: flags=--mount --map-root-user: attach to inferior

the test tries to run the 'unshare' application.  Sometimes though,
the application is present, but the set of flags used is not
supported (maybe due to restrictions on the local machine), so we see
behaviour like this:

  $ unshare --mount --map-root-user /bin/true; echo $?
  unshare: unshare failed: Operation not permitted
  1

Handle this case by first running 'unshare' with the same flags, but
using '/bin/true', if this fails then assume the flags are not
supported, and skip the test.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33108