test-coroutine: add performance test for nesting

The performance test will also check for nesting. It will do
a certain quantity of cycles, and each of one will do a depth
nesting process.

This is useful for benchmarking the creation of coroutines,
given that nesting is creation-intensive (and the other perf
test does not benchmark that).

Signed-off-by: Alex Barcelo <abarcelo@ac.upc.edu>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
