Tests: Add a check for color support when printing status

When printing the status of test is OK or FAIL, check if stdout is
attached to a terminal device. This way the output is not cluttered with
useless escape characters. Some use cases where we don't want colors:

$ ./sometest | less

$ ./sometest > a.log

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>