[GISel][UnitTest] Fix a bunch of tests that were not doing anything

After r368065, all the tests using GISelMITest must call setUp() before
doing anything, otherwise the TargetMachine is not going to be set up.
A few tests added after that commit were not doing that and ended up
testing effectively nothing.

Fix the setup of all the tests and fix the failing tests.

llvm-svn: 374595