Simplify TestHciLayer

Automatically record promises when enqueuing commands. This removes the
SetCommandFuture stuff that is confusing and bug-prone.

Test: le_advertising_manager gd unit tests
Bug: 247839849
Bug: 245578454
Merged-In: Ica34fcfbb79a245f477266b686df0b31354334f8
Change-Id: Ica34fcfbb79a245f477266b686df0b31354334f8
(cherry picked from commit fd74320a)