[knx] Remove obsolete public modifier in tests (#13776)

[knx] Remove obsolete public modifier in tests (#13776)

Since JUnit Jupiter test classes and methods do not need to be `public`.

It is recommended to omit the `public` modifier.
See https://junit.org/junit5/docs/current/user-guide/#writing-tests-classes-and-methods

"Class and method visibility
Test classes, test methods, and lifecycle methods are not required to be
`public`, but they must not be `private`.

It is generally recommended to omit the `public` modifier for test
classes, test methods, and lifecycle methods unless there is a technical
reason for doing so (...)"

Signed-off-by: Thomas Traude <t-riggs@gmx.net>