improve phpunit-bootstrap

[simonLeary42]

• replace switchUser / getUser* with becomeUser*
• add assertions into each becomeUser

this will make tests simpler to write since they don't need to sanity check the initial state of the database like this:

        switchUser(...getUserIsPIHasNoMembersNoMemberRequests());
        $pi_group = $USER->getPIGroup();
        $gid = $pi_group->getPIUID();
        $this->assertTrue($USER->isPI());
        $this->assertEquals([], $pi_group->getGroupMembers());
        $this->assertEquals([], $SQL->getRequests($gid));