suite: add --subset option to allow us to schedule subsets of a suite

suite: add --subset option to allow us to schedule subsets of a suite

First, the old logic tended to do the following:

[0,1,2,3] x [a,b,c,d] -> [(0, a), (0, b), (0, c), ...]

which is a bummer since if we only schedule the first 1/4,
we only get combinations with 0.

Instead, we want:

[0,1,2,3] x [a,b,c,d] -> [(0, a), (1, b), (2, c), ...]

Second, the old logic tended to do the following:

[0,1,2,3] + [a,b,c,d] -> [0,1,2,3,a,b,c,d]

Which is a bummer since we need to all the way through the first
set before we get examples from the second.

Instead, we want:

[0,1,2,3] + [a,b,c,d] -> [0,a,1,b,2,c,3,d]

Lastly, if we have:

[0,1,2] + ([0,1,2],[a,b,c])

and want to schedule 1/3 of the suite, we don't want to
just schedule 0, we probably want to schedule all of
[0,1,2] and 1/3 of the second combo.  Thus, we want to
detect such cases and multiply out [0,1,2] into
[0,1,2,0,1,2,0,1,2].

Signed-off-by: Samuel Just <sam.just@inktank.com>