[Documentation] Expand on testing variants.
[Documentation] Expand on testing variants.

The testing documentation appears to be from an era when the only kind
of tests were the lldbsuite python tests. This patch adds a short
description of the unittests and LIT tests and how to run them.

Patch by: Lawrence D'Anna

Differential revision: https://reviews.llvm.org/D67943

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@372797 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb