Add variable CTEST_CHANGE_ID to configure the setting. This allows
CTest clients to give CDash information about what change is being
tested so that CDash can take actions to report the results (e.g. to a
pull request page).
Add a TestLoad setting to CTest that can be set via a new --test-load
command-line option, CTEST_TEST_LOAD variable, or TEST_LOAD option to
the ctest_test command. Teach cmCTestMultiProcessHandler to measure
the CPU load and avoid starting tests that may take more than the
spare load currently available. The expression
<current_load> + <test_processors> <= <max-load>
must be true to start a new test.
Co-Author: Zack Galbreath <zack.galbreath@kitware.com>