Fix roundabouts with footway exits

[oxidase]

Issue

Assertion in #4129 occurs due to overwriting EnterRoundabout instruction at

osrm-backend/src/engine/guidance/post_processing.cpp

Lines 171 to 179 in 62b6c47

	if (!guidance::haveSameMode(exit_step, prev_step))
	{
	BOOST_ASSERT(leavesRoundabout(exit_step.maneuver.instruction));
	prev_step.maneuver.instruction = exit_step.maneuver.instruction;
	if (!entersRoundabout(prev_step.maneuver.instruction))
	prev_step.maneuver.exit = exit_step.maneuver.exit;
	exit_step.maneuver.instruction.type = TurnType::Notification;
	step_index--;
	}

, so enterRoundabout could correspond to another roundabout

Tasklist

• add regression / cucumber cases (see docs/testing.md)
• fix the issue
• review
• adjust for comments

Requirements / Relations

Link any requirements here. Other pull requests this PR is based on?

[oxidase]

@TheMarex the issue is related to #3766 The result for the query

EDIT: please check expectation values because different modes change exits numbering and instructions for a,d, a,f, a,h announce always 1st exit

[TheMarex]

Behavior of emitting an additional notification instruction makes sense.