Add tests for internal/snmp

## Feature Request

Opening a feature request kicks off a discussion.

### Proposal:

Add tests for [internal/snmp](https://pkg.go.dev/github.com/influxdata/telegraf/internal/snmp) package:

- [ ] `ClearCache`
- [ ] `GetIndex`
- [x] `LoadMibsFromPath` #10551
- [ ] `NewWrapper`
- [ ] `SetAgent`
- [ ] `SnmpTranslateCall`
- [x] `TrapLookup` #10430

Some of these are already covered by tests in plugin/inputs/snmp, but best is to test them at the correct place.

### Current behavior:

Changes in code do change behaviour of some plugins.

### Desired behavior:

Tests prevent changes in behaviour to the end-users.

Start by reading the internal/snmp package and the existing tests under plugin/inputs/snmp, since the issue notes that some functions are already covered there. Add package-level tests for the unchecked functions: ClearCache, GetIndex, NewWrapper, SetAgent, and SnmpTranslateCall. The work is done when these behaviors are covered without duplicating the completed LoadMibsFromPath and TrapLookup items.

Written by the indexing model from the issue text.