Added unit tests in pkg/es/config (jaegertracing#5819)

## Which problem is this PR solving?
Part of: jaegertracing#5068

## Description of the changes
- This commit adds tests for the `pkg/es/config` package.

## How was this change tested?
- `make test`

## Checklist
- [x] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [x] I have signed all commits
- [x] I have added unit tests for the new functionality
- [x] I have run lint and test steps successfully
  - for `jaeger`: `make lint test`
  - for `jaeger-ui`: `yarn lint` and `yarn test`

Signed-off-by: nabil salah <[email protected]>
Signed-off-by: Jared Tan <[email protected]>