gdbsupport/selftest: Replace for_each_selftest with an iterator_range

Remove the callback-based selftests::for_each_selftest function and use
an iterator_range instead.

Also use this iterator range in run_tests so all iterations over the
selftests are done in a consistent way.  This will become useful in a
later commit.

Change-Id: I0b3a5349a7987fbcb0071f11c394e353df986583