Support line_no option in "one-liner-syntax" specs

What does this MR do and why?

The full_description of one liner syntax is not expected:

# The real message
trial_registrations/new.html.haml is expected to have visible link "Start a Self-Managed trial" with href "https://about.gitlab.com/free-trial/#selfmanaged/"

# `example.full_message`
trial_registrations/new.html.haml 

So we need use :line_number to locate.

Usage

#- file_path: 'ee/spec/views/trial_registrations/new.html.haml_spec.rb'
#  description:
#    - 'trial_registrations/new.html.haml is expected to have visible link "Start a Self-Managed trial" with href "https://about.gitlab.com/free-trial/#selfmanaged/"'
- file_path: 'ee/spec/views/trial_registrations/new.html.haml_spec.rb'
  line_number: 21

Notice

This is a temporary solution, we will remove the 'line_no' option in the issue: #1655

/cc @jeremywu