docs: explain why interdependent tests are bad

[TrueFurina]

Summary

Per #683: extends the "Data handling" section of Testing-your-application.md to call out the test-order anti-pattern (one test filling the database for later tests).

Changes

• docs/site/Testing-your-application.md: new "### Avoid interdependent tests" subsection under Data handling, with:
  • the anti-pattern example (create-todo test feeding the delete-todos test)
  • why it is bad: cascade failures with misleading errors when the first test fails; tests can no longer run individually (it.only(), mocha -g)
  • the fix: independent tests with beforeEach() setup and test-data builders

Example source

Adapted from the acceptance tests referenced in the issue (strongloop/loopback4-example-getting-started PR #2).