gdb/testsuite: make more use of mi-support.exp

Building on the previous commit, now that the breakpoint related
support functions in lib/mi-support.exp can now help creating the
patterns for thread specific breakpoints, make use of this
functionality for gdb.mi/mi-nsmoribund.exp and gdb.mi/mi-pending.exp.

There should be no changes in what is tested after this commit.

Reviewed-By: Pedro Alves <pedro@palves.net>