end-to-end tests: use Go build tags instead of `MODE` environment variable

## Task summary

The standard is to use Go build tags for separate test types.
We should migrate from the global variable `MODE` reading from the environment variable `MODE` (`rpc`, `stress`, etc.) and use Go build tags instead like we do for integration tests.

Acceptance criteria:

- Remove `MODE` global variable in `tests/utils/gossamer_utils.go`
- Add `//go:build ` to each end to end test files
- Update CI workflows to use `go test ... -tags ` instead of `MODE= go test ....`