When you are in a test environment, you can expect that things are not completely functional. It does not mean that the environment is completely broken.
pecific behaviour can be expected from an extension. From Liquid Threads it can be expected that you can reply to a thread. It is possible to create a test for this.
The benefits are obvious; no broken code goes live in the test environment, the test environment does not get corrupted and the people who test are not bothered by functionality that is broken. Essentially some time needs to be invested to write a test up front. The return on investment is huge because it is not only the developers time that is saved, no need to restore the test environment, but also the time of the people who test.
The time of the people who test is typically not valued. It is however a sign of respect to the testers that obvious tests are performed first before they are exposed to new functionality.