Update testing documentation with more consistent terminology.

Update testing documentation with more consistent terminology.

All of LLVM's testing features are now referred to as the "Testing
infrastructure", the DejaGNU tests are just that, and the whole program tests
are referred to as the "test suite".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52674 91177308-0d34-0410-b5e6-96231b3b80d8