will do a full testsuite run (more thorough, but takes a lot longer). You should expect that there are no test case failures for the "fast" mode as that is a quality level that all GHC developers are expected to maintain when they check in code. There will usually be some test case failures for the full testsuite run though.
The run time of the testsuite can be reduced by running it with multiple threads. For instance if your machine has 8 cores, the following is probably appropriate:
$ make fulltest THREADS=6
## Using the Testsuite
-[Running the testsuite](building/running-tests/running)