This is to avoid scope issues with CTEST_RUN_CURRENT_SCRIPT. If
ctest_start() is called within a function scope, the value of
CTEST_RUN_CURRENT_SCRIPT that it sets doesn't make it to the global
scope. With this change, ctest_start() no longer sets
CTEST_RUN_CURRENT_SCRIPT, and instead sets a field directly in
cmCTestScriptHandler. The old behavior of CTEST_RUN_CURRENT_SCRIPT
has also been kept for projects and tests that rely on setting it.