Add good full coverage beacon API tests

[dapplion]

What is your question?

Current tests for the beacon API are of sub-optimal quality:

• Very limited coverage
• No assurance that all and future new endpoints are tested
• Require import stubbing
• Extensive use of brittle stubs: which require big diffs when modifying unrelated cost.

In my subjective opinion the current test suite has a very high cost of maintenance with very little return in ensuring the API is correct.

Proposed solution

Proper tests for a the beacon API should:

• Have full coverage
• Ensure programmatically that all endpoints current and future are tested
• Be tested against a full BeaconChain instance without stubbing and with real data. We are already doing this for BeaconChain validation and such data is extremely fast to produce and to run tests against.

[dapplion]

More disabled tests

lodestar/packages/lodestar/test/unit/api/impl/beacon/state/fork.test.ts

Lines 9 to 10 in 5d086b5

	// TODO remove stub
	describe.skip("beacon api impl - state - get fork", function () {

lodestar/packages/lodestar/test/unit/api/impl/beacon/pool/pool.test.ts

Lines 25 to 26 in 5d086b5

	// TODO remove stub
	describe.skip("beacon pool api impl", function () {

lodestar/packages/lodestar/test/unit/api/impl/debug/index.test.ts

Lines 20 to 21 in 5d086b5

	// TODO remove stub
	describe.skip("api - debug - beacon", function () {

lodestar/packages/lodestar/test/unit/chain/factory/block/blockAssembly.test.ts

Lines 21 to 22 in 5d086b5

	// TODO remove stub
	describe.skip("block assembly", function () {

[wemeetagain]

We now have oapi schema compliance tests that ensure validity of request/response data shapes and proper support of headers, status codes, etc. This is a good halfway solution.
Closing this for now