Improve E2E test performance by using waits more efficiently

In https://gitlab.com/gitlab-org/gitlab-ce/issues/46074 we discussed changing default_max_wait_time to 0 but decided that the problem we were actually trying to solve was that we were forcing Capybara to wait unnecessarily.

Proposal

• Add wait/predicate/performance best practices to QA docs
• Identify the places where we're forcing Capybara to wait, and either use predicate methods (e.g., has_element?) appropriately or eliminate them entirely.
• Review usage of predicate methods to determine if they should be assertions (i.e., should an exception be raised if the predicate returns false?)

For example:

def has_merge_options?
  has_css?(element_selector_css(:merge_moment_dropdown))
end

def merge_immediately
  if has_merge_options?
    click_element :merge_moment_dropdown
    click_element :merge_immediately_option
  else
    click_element :merge_button
  end
end

Every time has_merge_options? returns false there a delay that doesn't need to occur. We should know whether a test needs to use the dropdown or not, so there's no need for has_merge_options?.

/cc @gl-quality