Added tests for get_speed_mph()

[yashasvid]

Title

Added test for get_speed_mph()

Description

The purpose of this PR is to improve test coverage for gpspi module function get_speed_mph()

Types of Changes

Put an x in the boxes that apply

• [x] Feature (non-breaking change which adds functionality)
• [ ] Bug Fix (non-breaking change that fixes an issue)
• [ ] Breaking Change (feature/fix that causes existing features to not work as expected)
• [ ] Documentation

Checklist

• [x] I have read the [contribute]contributing doc
• [x] Classes, scripts, and environment variables follow existing naming convention
• [x] Lint and Unit tests pass locally
• [x] New features on hardware have been tested on a local Raspberry Pi
• [x] Mention new programs/binaries if any must be installed along with this change
• [x] Mention new environment variables if any have been added to hardware/env file
• [x] Please make sure test coverage does not drop. If it does, please explain the reasons.
• [x] Any new required python modules are added to the requirements.txt