Solving conf_info from BR inherited by consumers' test_package

[lasote]

Changelog: Fix: Fixed a bug when getting the values from the self.conf in the conanfile of a test_package with build_requirements declaring the self.conf_info.
Docs: omit

The issue is in conan 1.x , the test_package conanfile cannot read (loses) the conf inherited from the build requires.
It works for 2.x BUT this is a conan-center-index recipes migration issue so we should provide the fix.

Closes #11975

[memsharded]

I'd say this is looking good, but there are a bunch of tests broken, it is not evident what is happening, should be investigated

[lasote]

I'd say this is looking good, but there are a bunch of tests broken, it is not evident what is happening, should be investigated

Yes, I saw. I think I shouldn't override the conan_file.conf. attribute but append it, to avoid possible None values being assigned and overwriting the values for the profile. I'll investigate.