Eliminate custom fast spec helper scripts, refactor fast_spec_helper

Issues:

• Eliminate custom *_fast_spec_helper for Web IDE... (#473074 - closed)
• Use git-grep or git-ls-files instead of find in... (#473195 - closed)

What does this MR do and why?

Addresses these follow-up issues:

• Eliminate custom *_fast_spec_helper for Web IDE... (#473074 - closed)
• Use git-grep or git-ls-files instead of find in... (#473195 - closed)

Changes made

1. Eliminates custom fast spec helper scripts in Remote Development Workspaces and Web IDE domains
2. Other associated cleanup and refactors to the common fast_spec_helper, including support for adding domain-specific require statements and RSpec configuration to the common fast_spec_helper.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. scripts/remote_development/run-smoke-test-suite.sh should still pass
2. scripts/run-fast-specs.sh should still pass
3. RUN_ALL_FAST_SPECS_INDIVIDUALLY=true scripts/run-fast-specs.sh should still pass
4. CI should still pass with pipeline:run-all-rspec and pipeline:run-as-if-foss labels set