Ok, would you mind if you give some examples to different explanations.
Well, each religion typically has numerous subsets, so saying any sweeping example doesn't really work for any religion as a whole. But I'll give numerous examples I've seen.
-Hindus, in general, believe that God is panentheistic, meaning that everything is within god. Who would god be testing? Itself? Beings in this sort of worldview are proposed to exist in a state of illusion which causes suffering, and over time they free themselves from the illusions and join or rejoin with god, either partially or completely.
-Buddhists often exclude any notions of deities, especially creator deities. Things aren't a test, they just "are". Suffering is a result of craving.
-I'm not really sure about Jews, but most that I've talked to don't really view life as a test. I'm not really sure about Sikhs or Bahai's either. As far as I've seen, the concept of a test is not a major aspect of their worldviews, if its included at all.
-Christians sometimes include the concept of a test, but for the most part their answer to why suffering or imperfection exists has to do with original sin or Satan, and that things were originally made by god to be good.
-Some various syncretic or new age or pagan religions propose a variety of reasons, such as higher selves wishing to experience all manner of different things on the various spectra of pain and pleasure, suffering and joy, weakness and power, malevolence and benevolence, scarcity and abundance, for spiritual development.
-The ones that I encounter that usually believe life is a test are Muslims and some subsets of other Abrahamic religions.
First define flawed brains ?
A brain that is not ideal. Kind of a broad subject. It can include mental disorder, lack of intelligence, improper connections, genetic problems, chemical imbalances, etc.
Second, you talked about brain abilities and how it differs, Ok I don't see how this would be relevant. If you would assume that is because of different brain abilities that we have ,then the test is invalid, then how would you explain the variety of personality characteristics, traits of the followers of one faith. if that is the case a group A with certain qualities would follow religion 1 which is not true in practice.
It's relevant because what I'm saying is that there isn't any free agent to test. Biologically, the flaws of existence extend to the flaws of human perception, action, thoughts, consciousness, and choices.
Life developed on this planet billions of years ago, and complex life has existed for hundreds of millions of years. Humans are a rather recent addition.
Before we even got here, 99% of all species that have ever existed went extinct. Some catastrophic natural events, such as asteroids or massive volcanoes, or ice ages, were responsible for the deaths of the majority of lifeforms on the planet at a given time, and also the natural hardship of the predator/prey model caused death and extinction.
What was being tested during these countless years where there existed imperfection, but no sapient beings?
Is it in the case of us or in the case of God, the testing is applicable to us since we don't really know the results so we should try our best to succeed.
We don't know, but if God is proposed to know, then why bother with the test?