COMMON: Moved extremum time definitions into common header

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: Minor change - replace ASSERT_ by EXPECT_

This is specific to GTest framework - it was a typo.
We always "expect" in our tests.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
SYNTH: Added definitions for modulator parameters

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
SYNTH: Integrated Vca (modulator) into the audio path

Fixes #57.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: Loosened a bit synthesizer click check due to the use of Vca

The modulator introduces minor amplitude changes that may alter a bit
our very simple click detection.
That's why the test is loosened here - still, that's small and the test
is still able to catch early major regression on audio quality.

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