tests: move specs from `tests/unit/**/*` into packages

[kellymears]

This is too big a process to hinge on a single PR. We'll take it on piecemeal until tests/unit is empty. Then we'll close this issue.

This PR lays the groundwork: https://github.com/roots/bud/pull/1623.

Tests should:

• be alongside their corresponding module
• import code directly from that module
• be named roughly in accordance with the pattern established in #1623.
  • @roots/bud-api/src/methods/assets/index.ts becomes:
    • @roots/bud-api/src/methods/assets/assets.method.ts
    • @roots/bud-api/src/methods/assets/assets.unit.test.ts

There are about 500 unit tests in the repo, we'll start by tracking progress on @roots/bud-api 😅:

@roots/bud-api

• [ ] @roots/bud-api/methods/alias
• [x] @roots/bud-api/methods/assets (#1623)
• [x] @roots/bud-api/methods/bundle (#1623)
• [x] @roots/bud-api/methods/config (#1623)
• [x] @roots/bud-api/methods/define (#1623)
• [ ] @roots/bud-api/methods/devtool
• [ ] @roots/bud-api/methods/entry
• [ ] @roots/bud-api/methods/experiments
• [ ] @roots/bud-api/methods/externals
• [ ] @roots/bud-api/methods/hash
• [ ] @roots/bud-api/methods/minimize
• [ ] @roots/bud-api/methods/persist
• [ ] @roots/bud-api/methods/provide
• [ ] @roots/bud-api/methods/proxy
• [ ] @roots/bud-api/methods/runtime
• [x] @roots/bud-api/methods/serve (#1623)
• [ ] @roots/bud-api/methods/splitChunks
• [ ] @roots/bud-api/methods/template
• [ ] @roots/bud-api/methods/use
• [ ] @roots/bud-api/methods/watch
• [x] @roots/bud-api/service (#1623)

@roots/bud-server

• [x] all existing tests (#1606)