Re-enable Slasher E2E Test

[brightredchilli]

What does this PR do? Why is it needed?
Bugfix, Re-enables the Slasher E2E Test.

In a nutshell, slashing was flaky because the other beacon node doesnt get a chance to propose. See #12415 (comment) for more details.

Which issues(s) does this PR fix?
Fixes #12415

Other notes for review

Tests ran 50 times locally:

[57 / 107] 1 / 1 tests; Testing //testing/endtoend:go_default_test (run 50 of 50); 115s darwin-sandbox
Target //testing/endtoend:go_default_test up-to-date:
  bazel-bin/testing/endtoend/go_default_test_/go_default_test
INFO: Elapsed time: 5804.892s, Critical Path: 120.36s

There are 2 commits to this PR:

1. Commit 1 introduces the bare minimum fix by copy-pasting and duplicating the relevant parts in the test
2. Commit 2 goes a bit further by refactoring and keeping the evaluators/slashing.go file cleaner. I didn't refactor the double block evalulator. This commit can be discarded or modified as necessary.

Also, I want to point out a couple of things that are different from before:

1. This test used to double attest for the slot at currentslot-1, but that is no longer possible due to the chanage here. Double attesting for the current slot works just fine.
2. Because the P2P network filters out messages that it has seen before, the double attestation sent to each node must vary slightly - I did this by attaching some random bytes to the block root, but something simpler like a counter could work too, or just taking the arg directly from the test callsite.

On another side note, I was able to run tests locally successfully with SecondsPerSlot reduced from 10 to 3. Might be worth looking into to speed up the integration tests.

[CLAassistant]

All committers have signed the CLA.

[nisdas]

@brightredchilli Lint fails here

  testing/endtoend/evaluators/slashing.go:127: File is not `gofmt`-ed with `-s` (gofmt)
  
  testing/endtoend/evaluators/slashing_helper.go:20: File is not `gofmt`-ed with `-s` (gofmt)
  	valClient eth.BeaconNodeValidatorClient

Please run gofmt on the branch

[brightredchilli]

@nisdas ran gofmt on the files that i changed. gofmt also changes some other files in the repo but i didnt check those changes in.