ENH: Auto-enable CTest.UpdateCVS test on Windows
The test needs to create a cvs repository with 'cvs init', but the CVSNT client on Windows needs 'cvs init -n' to avoid administrator access. Previously we required users to explicitly enable CTEST_TEST_UPDATE_CVS to activate the test on Windows. This teaches the test to use the '-n' option when necessary. Now we can enable the test in all cases except when trying to use a cygwin cvs.exe without cygwin paths.