Macros to execute compiled program don't force it to quit when test exits

If main has an infinite loop, it'll just keep running as a process in the background even after the test has failed.

This became obvious in Paint Milestone 4 tests when a window was displayed. Here's how I solved it (although in this case I was looking for a timeout):

TEST(PaintProgramTest, StartShowsImage) {
std::chrono::time_point start, end;
start = std::chrono::system_clock::now();
system("timeout 1s ./main");
end = std::chrono::system_clock::now();
std::chrono::duration elapsed_seconds = end - start;
std::chrono::milliseconds ms =
std::chrono::duration_cast(elapsed_seconds);
if (ms.count() <= 900) {
FAIL() << " main() should call PaintProgram::Start() which calls "
"Image::ShowUntilClosed().";
}
}

Start by locating the test macros that execute compiled programs and the TEST case shown in the report. Reproduce the failure with a program whose main has an infinite loop, then inspect how the macro launches and observes that process. Done means a failed or exiting test does not leave the compiled program running in the background.

Written by the indexing model from the issue text.