Introduce the term 'test spec' in the API/CLI

[jorgeepc]

Context: as part of v0.7 we want to improve the UX/UI around the Test Run Details page and we are including the new term Test Spec instead of Assertions. We want to have the same terminology in the API/CLI.

AC1 Update the test file to include the term specs instead of testDefinition

name: my test
trigger:
   type: http
specs:
- selector: span[...]
  assertions:
  - tracetest.span.duration < 100ms

AC2 Update API references to match the new term spec