[lldb] Introduce CMake variable LLDB_ENFORCE_STRICT_TEST_REQUIREMENTS

The goal of this patch is to add the ability for the CMake configure to
fail when some optional test dependencies are not met. LLDB tries to be
flexible when test dependencies are not present but there are cases
where it would be useful to know that these dependencies are missing
before we run the test suite.

The intent here is to apply this setting on CI machines and make sure
that they have useful optional dependencies installed. We recently hit a
case where some CI machines were timing out while running the test suite
because a few tests were hanging. With this option, we'll be able to
know if the machine does not have psutil installed so we can install it
and avoid the timeout scenario altogether.

rdar://103194447

Differential Revision: https://reviews.llvm.org/D146335