build(test): wire pnpm test to vitest, drop redundant test:vitest CI step

src/package.json:
- test: was mocha --import=tsx --recursive ...; now `vitest run` (vitest.config
  already includes tests/backend/specs, tests/backend-new/specs, and
  tests/container/specs)
- test-utils: vitest run with --testTimeout 5000
- test-container: vitest run tests/container/specs/api
- test:vitest renamed to test:watch (watch mode for local dev)

.github/workflows/backend-tests.yml:
- removed the redundant 'Run the new vitest tests' step from all 4 jobs since
  pnpm test now runs vitest itself

.github/workflows/backend-tests.yml

@@ -68,9 +68,6 @@ jobs:
       -
         name: Run the backend tests
         run: pnpm test
-      - name: Run the new vitest tests
-        working-directory: src
-        run: pnpm run test:vitest
 
   withpluginsLinux:
     env:
@@ -137,9 +134,6 @@ jobs:
       -
         name: Run the backend tests
         run: pnpm test
-      - name: Run the new vitest tests
-        working-directory: src
-        run: pnpm run test:vitest
 
   # Windows tests only run on push to develop/master, not on PRs
   withoutpluginsWindows:
@@ -189,9 +183,6 @@ jobs:
         name: Run the backend tests
         working-directory: src
         run: pnpm test
-      - name: Run the new vitest tests
-        working-directory: src
-        run: pnpm run test:vitest
 
   withpluginsWindows:
     env:
@@ -267,6 +258,3 @@ jobs:
         name: Run the backend tests
         working-directory: src
         run: pnpm test
-      - name: Run the new vitest tests
-        working-directory: src
-        run: pnpm run test:vitest