[FEATURE] Testing other DNS servers

## Feature Summary
Please add an option for the user to test other DNS servers than the preconfigured ones.

## Problem Statement
Sometimes the user like me know additional DNS servers that might come in handy and want to test those which are not included in the program script.

## Proposed Solution
Adding a tab where we can input a list of preferred DNS servers to add for testing.

## Alternatives Considered
Maybe consider a tab where we can test any server we want one at a time.

## Additional Context
Really great app and I hope it gets better by time and by listening to user suggestions. Keep up the good work!