14d0acb refactor: run e2e tests with independent isolated servers too. (Jose Celano)
1df870b refactor: extract logic for setting up test envs (Jose Celano)
36f17f1 tests: panic when E2E server env is not running (Jose Celano)
bce946f refactor: removed unneeded intermediary var (Jose Celano)

Pull request description:

  We are running all E2E with the same application instance. The process is as follows:

  1.- Start docker containers with E2E env.
  2.- Run E2E tests.
  3.- Stop containers.

  After merging this [PR](#129), we can run an isolated app instance for each test.

  This PR allows E2E tests to be executed with isolated app instances:

  - Without using docker.
  - And in a way, we can mock services, setting the initial test state easier, etcetera.

  For now, we can run most of the E2E tests twice: with the shared docker instance or with the isolated app instances. YOu can change the behaviour with an env var.

  With the shared app (docker):

  ```text
  TORRUST_IDX_BACK_E2E_SHARED=true cargo test
  ```

  With isolated app instance (only rust):

  ```text
  cargo test
  ```

  Some tests cannot be executed in an isolated env because they require a running tracker. We could change them to use a mock.

  In general, I would try to keep only a few E2E tests using the real tracker and most of them using the mock. In the future, we could differentiate them using a different folder. Right now, the ones that require the running tracker have this:

  ```rust
  if !env.provides_a_tracker() {
     return
  }
  ```

  to skip the test when the test env does not provide a running tracker.

ACKs for top commit:
  josecelano:
    ACK 14d0acb

Tree-SHA512: 3ab562785938548caf238fd91d6894d0a1dc8e02eece10e157286fc9cb2950f9fdf5393c97b696ee5bde24577191c268fd95764eee5604f51b2ab314154f3125