[test] Allow setting extra environment for running tests.

[drodriguez]

In some situations, in order to set the correct environment during some
tests, extra environment has to be provided to the LLVM Lit invocations.
It seems that the preferred way is using cmake -E env to be
cross-platform.

The environment can be set using a cached variable so it can be set from
CMake cache files, or passing parameters to the CMake configuration
step.

/cc @rmaz

[drodriguez]

@swift-ci please smoke test

[gmittert]

Changes look reasonable to me. What kind of tests need this additional environment?

[drodriguez]

@swift-ci please test Windows platform

[drodriguez]

More background for the changes:

The Python tests require importing six. Normally one will have this installed on the system Python, or inside a virtualenv. Other environments that try to keep builds isolated will not allow installing dependencies in the system Python, or provide virtualenv. However, using PYTHONPATH to point to a local copy of six is enough for the tests.

Additionally the build system tests require cmake to be available in the path. Some environments do not have it in the path (one can invoke CMake with a full path). Adding cmake to the PATH temporarily for tests is possible this way, without adding it to more parts of the build.