How much testing is done on each pull request before it is merged? Frequently after one pull request is merged, multiple other pull requests follow to clean up a problem created by the first PR

Recent example:
https://github.com/bitcoin/bitcoin/pull/7957

https://github.com/bitcoin/bitcoin/pull/8164
https://github.com/bitcoin/bitcoin/pull/8171

Normally no real harm is done. Is this no big deal or a sign that more testing should be done before Bitcoin pull requests are merged?
Each pull request usually isn't that big, so it's not hard to look through every line of code. Each pull goes through meticulous testing, in case of any malicious code or an unintended flaw. Why do you think it takes usually more than one month after the pull is made to get added, even though there is alot of discussion in the comments?