[Test] Fix mocked functions.

[eloycoto]

If any function is mocked used stub need to be reverted at some point,
doing something like this

before_each(function()
  stub(foo, "validate", function() return false end)
end)

after_each(function()
  foo.validate:revert()
end)

it("Validate stub foo", function()
  assert.same(foo.validate(), false)
end)

In APICast all ngx variables are reset on ngx_helper.lua but some
methods like ngx_variables or backend_client are not reset and the
mock function is in there for all other test.

The test in spec/policy/ngx_variable_spec.lua validates that
ngx_variable.available_context is working correctly, due some other
test can mock this function the test was inestable over the last days.

This commit creates a way to define a cleanup for mocked functions that
are not part of the ngx class.

[mikz]

All mocks/stubs should be reverted automatically. If they are applied in some global context (not after/before_each), then they would not be reverted between tests.

I see this change as a bandaid and yet another way to rollback changes. Why we can't just rely on the rollbacking the stubs?

[eloycoto]

@mikz I'm ok to apply the :revert() function on the after_each functions, I think that it's a bit clear and It makes things easier to read and debug.

Last week, I had a chat with @davidor and we agreed that we should use the same way that we use for ngx.* methods.

[mikz]

@eloycoto but that should not be needed at all. There are global after/before hooks that do that for all registered stubs. There should be no reason to manually revert them because it would be really error-prone.

https://github.com/3scale/APIcast/blob/master/spec/luassert_helper.lua (was significantly refactored in #799).

[eloycoto]

Will check, thanks!

[eloycoto]

Close this one due to is not the correct solution, the correct one is on #1042