move assertion helper into base unit test
rename file

line_extension_singular contains tests for /line/id/extension.
A new series of tests will be written for the new association URL
add missing prerequesites to extension scenarios
add scenarios for listing all extensions associated to a line
add resource for listing all extensions associated to a line
add scenarios for associating extension with line
add routes for associating extension with line
add scenarios for dissociating extension from line
add routes for dissociating an extension from a line
return 404 if line_id or extension_id does not exist

Looks hackish mainly because I haven't had time to
continue cleaning the exceptions in the DAO
add missing steps for associating a user with a line
fix error messages returned by RESTAPI
fix extension number in line_extension_collection test
Merge branch '5003_restapi_incall_final'