[cmake] Move the including of utils/unittests under LLVM_INCLUDE_UTILS instead of LLVM_INCLUDE_TESTS.

This does not change anything by default since LLVM_INCLUDE_UTILS is already set
to TRUE by default. In addition, since LLVM_INCLUDE_TESTS => LLVM_INCLUDE_UTILS,
the only way that this can cause changes is in the case where LLVM_INCLUDE_UTILS
is set to TRUE, but LLVM_INCLUDE_TESTS is FALSE. In that case, building gtest is
not a huge cost.

The reason to do this is that without this change, one can not turn off
LLVM_INCLUDE_TESTS in downstream projects that also use gtest for unittests. It
also just in general makes more sense since LLVM_INCLUDE_UTILS gates FileCheck
and other utilities that are along the lines of gtest.

Additionally from talking with chandlerc, this was not done for any specific
reason, so there is no reason not to do it and lots of benefit to doing it.

llvm-svn: 276342