Implement tests for new staking endpoints

[fedekunze]

Summary

As for #1880, we originally left 2 endpoints commented since @faboweb and I agreed on having them for later. Handler logic is done, just need to create some tests.

Problem Definition

The new endpoints are the following and are needed to query each of the validators that a delegator is bonded to.

// x/stake/client/rest/query.go
GET /stake/delegators/{delegatorAddr}/validators // Query all validators that a delegator is bonded to
GET /stake/delegators/{delegatorAddr}/validators/{validatorAddr} // Query a validator that a delegator is bonded to

Proposal

Implement tests for delegatorValidatorsHandlerFn and delegatorValidatorHandlerFn respectively.

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned