gdb/testsuite: check that "info shared" and "info linker-namespaces" before running...

gdb/testsuite: check that "info shared" and "info linker-namespaces" before running don't crash

While writing my solib_ops C++ification series, I broke this, and it
didn't seem to be caught by the testsuite.  Add a test for those.

The exact message for "info linker-namespaces" varies depending on the
solib_ops of the target architecture (whether ops->num_active_namespaces
is nullptr or not).  For now, just accept any message (a crash will
still be caught).  A later patch in this series will make the message
consistent and update this test.

Change-Id: I6bce2ff317447bbf321fc9cbd2d42c3dcea0c683
Approved-By: Pedro Alves <pedro@palves.net>