ADSD: Fixed #59 (Envelop not beginning at zero)

This was due to the current value being generated was already changed
by internal computation.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: In Adsd tests, added a clean "tail" definition and using it

In this context the tail is the time duration during which the generator
output gets checked after the release.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MODULATORS: Added a timings check for Adsd envelop generator

Now per-sample precise, except for the decay (see #60).

Signed-off-by: Gama <misept.dieseneuf@gmail.com>