Round test duration seconds to nearest millisecond (3 decimal places)

[MattMcL4475]

Currently, the test duration is printed with 14 decimal places, like this:

test_name (2.43199897100011 seconds)

This PR is to round the test duration to the nearest millisecond (3 decimal places), to make it more readable and practical, like this:

test_name (2.432 seconds)

[alexott]

Nice addition!