[lldb] Restore default setting of LLDB_INCLUDE_TESTS in standalone builds

In 52f39853 the option LLDB_INCLUDE_TESTS was
moved above the inclusion of LLDBStandalone. This isn't a problem per-se, but
it changes the default value of LLDB_INCLUDE_TESTS in standalone builds.
LLDBStandalone explicitly sets LLVM_INCLUDE_TESTS to true, indicating that
for standalone builds this is considered the default behavior.
This patch restores said default behavior.

Differential Revision: https://reviews.llvm.org/D138237