gdb/testsuite: fix gdb.base/dlmopen-ns-ids.exp racy test

gdb/testsuite: fix gdb.base/dlmopen-ns-ids.exp racy test

The recently included gdb.base/dlmopen-ns-ids.exp test can sometimes
fail the call to get_integer_valueof when trying to check the namespace
ID of the fourth dlopened SO, for apparently no reason.

What's happening is that the call to get_first_so_ns doesn't necessarily
consume the GDB prompt, and so get_integer_valueof will see the prompt
immediately and not find the value the test is looking for.

To fix this, the test was changed so that we consume all of the output
of the command "info sharedlibrary", but only set the namespace ID for
the first occurrence of the SO we're looking for.  The command now also
gets the solib name as a parameter, to reduce the amount of output.

Co-Authored-By: Tom de Vries <tdevries@suse.de>
Approved-By: Tom de Vries <tdevries@suse.de>