Add `action_view.finalize_compiled_template_methods` config option

ActionView::Template instances compile their source to methods on the
ActionView::CompiledTemplates module. To prevent leaks in development
mode, where templates can frequently change, a finalizer is added that
undefines these methods[1] when the templates are garbage-collected.

This is undesirable in the test environment, however, as templates don't
change during the life of the test. Moreover, the cost of undefining a
method is proportional to the number of descendants a class or module
has, since the method cache must be cleared for all descendant classes.

As ActionView::CompiledTemplates is mixed into every
ActionView::TestCase (or in RSpec suites, every view spec example
group), it can end up with a very large number of descendants, and
undefining its methods can become very expensive.

In large test suites, this results in a long delay at the end of the
test suite as all template finalizers are run, only for the process to
then exit.

To avoid this unnecessary cost, this change adds a config option,
`action_view.finalize_compiled_template_methods`, defaulting to true,
and sets it to false in the test environment only.

[1] https://github.com/rails/rails/blob/09b2348f7fc8d4e7191e70e06608c5909067e2aa/actionview/lib/action_view/template.rb#L118-L126

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