test: add prompt setter to interactive helper

Interactive console test helper could be used for remote connection too,
for that purpose prompt needs to be changed accordingly to connection
type.

This patch introduces getter and setter for the prompt. Prompt is
configured per session, so it is advised to create a new session
for each test case (e.g. with before_each()).

NO_CHANGELOG=test helper change
NO_DOC=test helper change

(cherry picked from commit ed86a729)