(maint) Migrate ex to Pkg::Util::Execution

This commit migrates ex from 00_utils to a new module called
Pkg::Util::Execution. It also adds another method called success?, which
wraps the behavior of $?.success?, with the bonus of being more easily
stubbed/mocked in other tests. Some basic tests are included for ex. The
implementation of ex is moved from %x to `` to enable testability of the
method.
(maint) Update Pkg::Util::File for new Pkg::Util::Execution
(maint) Update 00_utils for Pkg::Util::Execution
(maint) Update Pkg::Util::File.mktemp to use Pkg::Util::Execution.ex instead of ``
(maint) Address order dependent test failures in Pkg::Util::Net

Because the net spec tests around ssh stub out ssh calls, there is no
guarantee that the $? object will be instantiated, which can cause tests
to fail if we try to get success? from nil. This commit updates the
tests to use the Pkg::Util::Execution.success? method, which is easily
stubbed out in tests. The implementation is also updates to use the same
method.