Contract test cleanup

What does this MR do and why?

This MR goes back and cleans up the codebase for contract tests. While writing new tests, more new cases came up such as GraphQL and async calls which resulted in a need to organize the codebase more to account for these. This MR covers the following:

• Updates consumer tests to use async calls and updating specs to use await for these async calls.
• Moves given/state attribute value to the fixtures for consistency between REST and GraphQL tests.
• Renames state files to use show_state, index_state, new_state, etc.
• Moves merge_request providers to be nested under show folder.
• Combines the 3 merge_request state files into one merge_request/show_state file.
• Updates rake task file path variables to reduce duplication.

Documentation updates will be done in #367745 (closed) and is awaiting this MR.

Related: #367744 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.