Configurable `T` parameter

[poszu]

The T parameter is hardcoded:

poet/shared/shared.go

Lines 14 to 16 in 7a0c2b9

	// T is the security param which determines the number of leaves
	// to be included in a non-interactive proof.
	T uint8 = 150

It's inconvenient in tests, where we wish to have short poet rounds (10 leaves is enough). Picking a short round time sometimes results in too short tree on slow runners

Acceptance criteria:

The T should be configurable, but default to 150.