Restored two unwarranted changes

UnmanagedImageTest: Color.White had accidentally been replaced with a
Color.FromArgb call. Restored to Color.White
LineSegmentTest: A number of tests had been commented away. These tests
are now included in build.
Removed redundant arguments

In LineSegmentTest.CollinearIntersectionPointTest, one of the test cases
contained 10 arguments where the argument list only requested 8. This
led to an error message, "Wrong number of arguments provided". To
overcome this error, the last two arguments in the specific test case
list have been removed.