CMake: generate check targets for lit suites without their own lit.cfgs

Currently our cmake generates targets like check-llvm-unit and
check-llvm-transforms-loopunroll-x86, but not check-llvm-transforms or
check-llvm-transforms-adce. This is because the search for test suites
only lists the ones with a custom lit.cfg or lit.local.cfg.

Instead, we can do something a little smarter - any directory under
test that isn't called Inputs or inside a directory called Inputs is a
test suite.

llvm-svn: 268806