[testsuite] gdb.base/dlmopen: Fix test name and use gdb_attach

One test name in gdb.base/dlmopen.exp changes from run to run
since it includes a process id:

    PASS: gdb.base/dlmopen.exp: attach 3442682

This is not convenient do diff gdb.sum files to compare test runs.

Fix by using gdb_attach helper function to handle attaching to the
process as it produce a constant test name.

While at it also check gdb_attach's return value to only run the
rest of the test if the attach was successful.

Approved-By: Simon Marchi <simon.marchi@efficios.com>