Add test coverage for the `injectRoute` API

[tony-sull]

What version of astro are you using?

1.0.0-beta.58

Are you using an SSR adapter? If so, which one?

None

What package manager are you using?

pnpm

What operating system are you using?

Mac

Describe the Bug

injectRoute doesn't have any test coverage yet!

Coverage needed

• astro dev
• astro build for SSR builds
• injecting basic pages, /_admin
• injected API endpoints
• injecting dynamic routes, /_admin/[userid]
• injecting dynamic routes with rest params, /_admin/[...params]

Link to Minimal Reproducible Example

NA

Participation

• [ ] I am willing to submit a pull request for this issue.