end-to-end tests: remove `TestMain` functions

## Task summary

Due to several prior refactors, this is now a straight forward removal task (10 minutes top).

The reasons behind this are to:

- Decouple tests in a same end-to-end tests package (i.e. `tests/stress`)
- Reduce the stack trace for test failures or panics
- Reduce code

Steps to achieve this:

1. Remove the global logger in `tests/utils/gossamer_utils.go`. This one is unused, so it's a straight-forward removal
2. Remove the now unused global variable `LOGLEVEL` from `tests/utils/common.go`
3. Remove the `tests/stress` and `tests/rpc` `TestMain` functions which are now no longer needed