test(coverage): improve and simplify tests overall

[AriPerkkio]

• Simplifies coverage tests
• Removes duplicate tests from C8 tests
• Creates generic tests cases used by both coverage providers. This increases covered functionality of C8 provider.
• Adds snapshot test for C8 provider. This should increase confidence and prevent from breaking stuff without noticing (e.g. fix: correctly resolve filename, when running code #2439).
• Reduces package.json scripts mess by using testing.mjs script - idea from vitest/test/css/testing.mjs

I started by debugging what those 12 package.json scripts do and the setup was quite unclear:

scripts run by "npm test" in earlier setup

- 1. pnpm run test:c8
  - 1.1. pnpm run test:thread:c8
    - 1.1.1. pnpm run coverage:thread:c8
      - THREAD=true vitest run --coverage.provider c8
    - 1.1.2. pnpm run coverage-test:c8
      - vitest -c vitest.config-c8-coverage.ts run
  - 1.2. pnpm run test:nothread:c8
    - 1.2.1. pnpm run coverage:c8
      - vitest run --coverage.provider c8
    - 1.2.2. pnpm run coverage-test:c8
      - vitest -c vitest.config-c8-coverage.ts run

- 2. pnpm run test:istanbul
  - 2.1. pnpm run test:thread:istanbul
    - 2.1.1. pnpm run coverage:thread:istanbul
      - THREAD=true vitest run --coverage.provider istanbul
    - 2.1.2. pnpm run coverage-test:istanbul
      - vitest -c vitest.config-istanbul-coverage.ts run
  - 2.2. pnpm run test:nothread:istanbul
    - 2.2.1. pnpm run coverage:istanbul
      - vitest run --coverage.provider istanbul
    - 2.2.2. pnpm run coverage-test:istanbul
      - vitest -c vitest.config-istanbul-coverage.ts run

Ends up running:

- THREAD=true vitest run --coverage.provider c8
- vitest -c vitest.config-c8-coverage.ts run
- vitest run --coverage.provider c8
- vitest -c vitest.config-c8-coverage.ts run

- THREAD=true vitest run --coverage.provider istanbul
- vitest -c vitest.config-istanbul-coverage.ts run
- vitest run --coverage.provider istanbul
- vitest -c vitest.config-istanbul-coverage.ts run

Now there is a simpler setup which should quite clearly indicate what actually happens with npm test.